python-pygtk: fix parallel compile issue

defs.c dependes on gdk-types.defs and gtk-types.defs. When parallel
compile, it may fails with:
"IOError: [Errno 2] No such file or directory: 'gtk-types.defs'"

Add them to dependences of defs.c to fix this issue.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Merge pull request #31 from drewmoseley/fix-build-race-in-python-pygtk

python-pygtk: fix parallel compile issue