add missing header guard to invoke.h (mr !459)

Backport of !457

* main/merge-requests/459:
  Merge branch 'add-missing-headerguard' into 'master'

GaudiKernel/GaudiKernel/invoke.h

+#ifndef GAUDIKERNEL_INVOKE_H
+#define GAUDIKERNEL_INVOKE_H
 #include <functional>
 #include <type_traits>
 #include <utility>
@@ -88,3 +90,4 @@ namespace Gaudi
     return detail2::INVOKE( std::forward<F>( f ), std::forward<ArgTypes>( args )... );
   }
 }
+#endif