Fix build against gtk+ older than 2.21.6.

The gdk_window_get_background_pattern() function copied from GDK and
introduced in commit 0839c100 has a small syntax error and uses a private
API (_gdk_drawable_ref_cairo_surface()). This patch imports the missing
API and fixes the syntax error.

https://bugzilla.gnome.org/show_bug.cgi?id=626583