MythZoneminder: Make sure the connection to the server is shutdown on…

… FE exit

This fixes a 'Not all threads were shut down properly' error when shutting
down the frontend.

Fixes #11340.

Signed-off-by: Daniel Thor Kristjansson <danielk@cuymedia.net>

mythplugins/mythzoneminder/mythzoneminder/main.cpp

@@ -166,5 +166,8 @@ int mythplugin_config(void)
     return 0;
 }
 
-
+void mythplugin_destroy(void)
+{
+    delete ZMClient::get();
+}