Replace gdk_pixbuf_unref with g_object_unref

Because gdk_pixbuf_unref is deprecated and for compatability with
gdk-pixbuf 2.21.4.
diff --git a/compositor/compositor.c b/compositor/compositor.c
index 27dd50d..d391fca 100644
--- a/compositor/compositor.c
+++ b/compositor/compositor.c
@@ -211,7 +211,7 @@
 
 	argb_pixels = malloc (height * width * 4);
 	if (argb_pixels == NULL) {
-		gdk_pixbuf_unref(pixbuf);
+		g_object_unref(pixbuf);
 		return NULL;
 	}
 
@@ -250,7 +250,7 @@
 		}
 	}
 
-	gdk_pixbuf_unref(pixbuf);
+	g_object_unref(pixbuf);
 
 	buffer = ec->create_buffer(ec, width, height,
 				   &ec->compositor.premultiplied_argb_visual,