glcolorconvert: fix missing printf arg for apple rectangular texture case

Found by -Wformat-nonliteral (to be re-enabled in a separate commit).
diff --git a/gst-libs/gst/gl/gstglcolorconvert.c b/gst-libs/gst/gl/gstglcolorconvert.c
index 6bd9197..734ebf4 100644
--- a/gst-libs/gst/gl/gstglcolorconvert.c
+++ b/gst-libs/gst/gl/gstglcolorconvert.c
@@ -1491,7 +1491,7 @@
      * for us */
     info->templ = &templ_REORDER;
     info->frag_body =
-        g_strdup_printf (info->templ->body, pixel_order[0], pixel_order[1],
+        g_strdup_printf (info->templ->body, "", pixel_order[0], pixel_order[1],
         pixel_order[2], pixel_order[3]);
     info->shader_tex_names[0] = "tex";
   } else {