Codechange: Use map.emplace() instead of map.insert(std::pair).

This avoids a copy of the pair into the map.

src/cargotype.cpp

@@ -99,7 +99,7 @@ void BuildCargoLabelMap()
 		/* Label already exists, don't addd again. */
 		if (CargoSpec::label_map.count(cs.label) != 0) continue;
 
-		CargoSpec::label_map.insert(std::make_pair(cs.label, cs.Index()));
+		CargoSpec::label_map.emplace(cs.label, cs.Index());
 	}
 }