Fix extra '-I' in docs/reference/libmx/Makefile.am
Fix extra '-I' in docs/reference/libmx/Makefile.am

pwithnall/mx,clutter-project/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx