Workaround gears depending on cairo-gl

Extremely hacky, but at least it doesn't stop the build anymore.
diff --git a/clients/window.h b/clients/window.h
index 341f602..79c7a82 100644
--- a/clients/window.h
+++ b/clients/window.h
@@ -51,10 +51,12 @@
 EGLDisplay
 display_get_egl_display(struct display *d);
 
+#ifdef HAVE_CAIRO_GL
 EGLImageKHR
 display_get_image_for_drm_surface(struct display *display,
 				  cairo_surface_t *surface);
 #endif
+#endif
 
 cairo_surface_t *
 display_create_surface(struct display *display,