psql: do file completion for \gx

This was missed when the feature was added.

Reported-by: Vik Fearing

Discussion: https://postgr.es/m/eca20529-0b06-b493-ee38-f071a75dcd5b@postgresfriends.org

Backpatch-through: 10

src/bin/psql/tab-complete.c

@@ -3886,7 +3886,7 @@ psql_completion(const char *text, int start, int end)
 		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_routines, NULL);
 	else if (TailMatchesCS("\\sv*"))
 		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_views, NULL);
-	else if (TailMatchesCS("\\cd|\\e|\\edit|\\g|\\i|\\include|"
+	else if (TailMatchesCS("\\cd|\\e|\\edit|\\g|\\gx|\\i|\\include|"
 						   "\\ir|\\include_relative|\\o|\\out|"
 						   "\\s|\\w|\\write|\\lo_import"))
 	{