fallbacksrc: Mark internal source property for custom sources as write/construct-only

It's only used during construction of the internal bin after all.
Keeping it readable would cause e.g. creating a pipeline graph trying to
read it, which is not implemented.

utils/fallbackswitch/src/fallbacksrc.rs

@@ -1948,7 +1948,7 @@ mod custom_source {
             "Source",
             "Source",
             gst::Element::static_type(),
-            glib::ParamFlags::READWRITE | glib::ParamFlags::CONSTRUCT_ONLY,
+            glib::ParamFlags::WRITABLE | glib::ParamFlags::CONSTRUCT_ONLY,
         )
     })];