Depend on gdk-pixbuf instead of gtk when gtk2

Origin: http://sources.gentoo.org/cgi-bin/viewvc.cgi/gentoo-x86/www-client/w3m/files/w3m-0.5.3-gdk-pixbuf.patch?revision=1.1

w3mimg/fb/fb_gdkpixbuf.c

@@ -6,7 +6,7 @@
 #include "config.h"
 #if defined(USE_GTK2)
 #include <glib-object.h>
-#include <gdk/gdk.h>
+#include <gdk-pixbuf/gdk-pixbuf.h>
 #endif
 #include <gdk-pixbuf/gdk-pixbuf.h>
 #include "fb.h"