gstreamer/log: Replace % with %% in the debug message

Otherwise gst_debug_log() will look for further arguments that don't
exist, and will likely crash or worse.

gstreamer/src/log.rs

@@ -150,7 +150,7 @@ impl DebugCategory {
                 module.to_glib_none().0,
                 line as i32,
                 obj_ptr,
-                fmt::format(args).to_glib_none().0,
+                fmt::format(args).replace("%", "%%").to_glib_none().0,
             );
         }
     }