Add missing include directory for Pythia8
Add missing include directory for Pythia8

A plain include_directories() was meant to be converted
into target_include_directories() in the previous commit,
but the second part was missing.

olifre/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,karies/root,karies/root,karies/root,root-mirror/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,root-mirror/root,olifre/root