build: Include gdk-pixdata.c when building GdkPixbuf-2.0.gir
build: Include gdk-pixdata.c when building GdkPixbuf-2.0.gir

After the split, this wasn't included here. We need this file because
it contains the annotations for gdk_pixbuf_new_from_inline - without
those we generate broken introspection metadata.

Closes #92

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf