multimedia/celluloid: unbreak with gtk4/X11=off after 51d62dcf1e3d

../src/celluloid-video-area.c:28:10: fatal error: 'gdk/x11/gdkx.h' file not found
#include <gdk/x11/gdkx.h>

^~~~~~~~~~~~~~~~

(cherry picked from commit 492fe7fbe48b568258866d01863de5eb0216b513)