Avoid side placement OOS by using std::stable_sort

Use of std::sort sometimes led to clients sorting placings with equal scores differently, leading to OOS when clients placed sides in different castles.

src/game_state.cpp

@@ -115,7 +115,7 @@ void game_state::place_sides_in_preferred_locations()
 		++side_num;
 	}
 
-	std::sort(placings.begin(),placings.end());
+	std::stable_sort(placings.begin(),placings.end());
 	std::set<int> placed;
 	std::set<map_location> positions_taken;