[Vala] Broken dependency in Gdk.Pixbuf

[Jiro Yamane <asotaro fubako com>]

Since gdkpixbuf is more or less separated from GDK, programs to use
gdkpixbuf should include gdk-pixbuf/gtk-pixbuf.h as well. However, C
codes generated by valac from Vala codes using the class Gdk.Pixbuf does
not include the header necessary for gdkpixbuf, so the compilation
fails. Is there a proper way to make valac include
gdk-pixbuf/gdk-pixbuf.h?

Thanks,
Jiro