gst: add missing define guard

If compiled with -Dgstreamer:gst_debug=false and we have
GST_REMOVE_DISABLED defined we will get the following compiler error:

```
[...]/libgstreamer-1.0.so.0.2100.0.p/gst.c.o: in function `gst_deinit':
[...]/gst/gst.c:1258: undefined reference to `_priv_gst_debug_cleanup'
[...] hidden symbol `_priv_gst_debug_cleanup' isn't defined
```

Add the missing define guard to avoid this.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2649>

subprojects/gstreamer/gst/gst.c

@@ -1268,10 +1268,12 @@ gst_deinit (void)
   GST_INFO ("deinitialized GStreamer");
   g_mutex_unlock (&init_lock);
 
+#ifndef GST_DISABLE_GST_DEBUG
   /* Doing this as the very last step to allow the above GST_INFO() to work
    * correctly. It's of course making the above statement a lie: for a short
    * while we're not deinitialized yet */
   _priv_gst_debug_cleanup ();
+#endif
 }
 
 /**