Consistent interface install location
With this PR, if -DHEPMC3_INSTALL_INTERFACES:BOOL=ON is used, the interfaces are installed where examples/Pythia8Example/CMakeLists.txt expects to find them when -DUSE_INSTALLED_HEPMC3:BOOL=ON is used.
With this PR, if -DHEPMC3_INSTALL_INTERFACES:BOOL=ON is used, the interfaces are installed where examples/Pythia8Example/CMakeLists.txt expects to find them when -DUSE_INSTALLED_HEPMC3:BOOL=ON is used.