Another small fix to build on macOS

Fixes this compilation error:

[ 81%] Building CXX object GaudiHive/CMakeFiles/GaudiHive.dir/src/PRGraph/PrecedenceRulesGraph.cpp.o
/Users/sftnight/key4hep/Gaudi/GaudiHive/src/PRGraph/PrecedenceRulesGraph.cpp:633:12: error: no matching member function for call to 'open'
    myfile.open( fileName, std::ios::app );
    ~~~~~~~^~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/fstream:1379:10: note: candidate function not viable: no known conversion from 'const boost::filesyste
m::path' to 'const char *' for 1st argument
    void open(const char* __s, ios_base::openmode __mode = ios_base::out);
         ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/fstream:1383:10: note: candidate function not viable: no known conversion from 'const boost::filesyste
m::path' to 'const std::string' (aka 'const basic_string<char, char_traits<char>, allocator<char>>') for 1st argument
    void open(const string& __s, ios_base::openmode __mode = ios_base::out);
         ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/fstream:1387:10: note: candidate function not viable: no known conversion from 'const boost::filesyste
m::path' to 'const filesystem::path' for 1st argument
    void open(const filesystem::path& __p, ios_base::openmode __mode = ios_base::out)
         ^
/Users/sftnight/key4hep/Gaudi/GaudiHive/src/PRGraph/PrecedenceRulesGraph.cpp:674:12: error: no matching member function for call to 'open'
    myfile.open( fileName, std::ios::app );
    ~~~~~~~^~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/fstream:1379:10: note: candidate function not viable: no known conversion from 'const boost::filesyste
m::path' to 'const char *' for 1st argument
    void open(const char* __s, ios_base::openmode __mode = ios_base::out);
         ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/fstream:1383:10: note: candidate function not viable: no known conversion from 'const boost::filesyste
m::path' to 'const std::string' (aka 'const basic_string<char, char_traits<char>, allocator<char>>') for 1st argument
    void open(const string& __s, ios_base::openmode __mode = ios_base::out);
         ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/v1/fstream:1387:10: note: candidate function not viable: no known conversion from 'const boost::filesystem::path' to 'const filesystem::path' for 1st argument
    void open(const filesystem::path& __p, ios_base::openmode __mode = ios_base::out)
         ^
2 errors generated.
make[2]: *** [GaudiHive/CMakeFiles/GaudiHive.dir/src/PRGraph/PrecedenceRulesGraph.cpp.o] Error 1

GaudiHive/src/PRGraph/PrecedenceRulesGraph.cpp

@@ -630,7 +630,7 @@ namespace concurrency {
 
   void PrecedenceRulesGraph::dumpPrecRules( const boost::filesystem::path& fileName, const EventSlot& slot ) {
     std::ofstream myfile;
-    myfile.open( fileName, std::ios::app );
+    myfile.open( fileName.c_str(), std::ios::app );
 
     // Declare properties to dump
     boost::dynamic_properties dp;
@@ -671,7 +671,7 @@ namespace concurrency {
   //---------------------------------------------------------------------------
   void PrecedenceRulesGraph::dumpPrecTrace( const boost::filesystem::path& fileName, const EventSlot& slot ) {
     std::ofstream myfile;
-    myfile.open( fileName, std::ios::app );
+    myfile.open( fileName.c_str(), std::ios::app );
 
     // Fill runtimes (as this could not be done on the fly during trace assembling)
     SmartIF<ITimelineSvc> timelineSvc = m_svcLocator->service<ITimelineSvc>( "TimelineSvc", false );