Support Vala-generated C header files in subdirectories

valac can generate C header files using its --header option. If the
--includedir option is also specified, the header file will be generated
in the given (sub)directory. git.mk previously assumed all generated
header files were in the current directory. This change fixes that so it
gets the full header file path from $(DIST_COMMON).
Merge pull request #13 from pwithnall/master

Support Vala-generated C header files in subdirectories