Changed a log domain to make [recall] shut up about...

..."recall list is empty when trying to recall" messages. These aren't
helpful for the wml author. [recall] is often used even though the
unit isn't supposed to be there.

src/game_events.cpp

@@ -1672,7 +1672,7 @@ WML_HANDLER_FUNCTION(recall, /*event_info*/, cfg)
 		const std::string player_id = (*resources::teams)[index].save_id();
 
 		if((*resources::teams)[index].recall_list().size() < 1) {
-			WRN_NG << "recall list is empty when trying to recall!\n"
+			DBG_NG << "recall list is empty when trying to recall!\n"
 				   << "player_id: " << player_id << " side: " << index+1 << "\n";
 			continue;
 		}