Fix glib build

Build fails with git tip (most likely due to https://code.google.com/p/go/source/detail?r=a70a32dc121a).

    $ go build
    # github.com/agl/go-gtk/glib
    ./glib.go:814: cannot use v.MainContext (type *C.GMainContext) as type *C.GMainLoop in function argument
    ./glib.go:814: cannot use _Cfunc_g_main_loop_ref(v.MainContext) (type *C.GMainLoop) as type *C.GMainContext in field value