Fix range-loop-construct warning.

src/generators/lua_map_generator.cpp

@@ -38,14 +38,14 @@ lua_map_generator::lua_map_generator(const config & cfg, const config* vars)
 {
 	lk_.load_core();
 	const std::array required {"id", "config_name", "create_map"};
-	for(const std::string& req : required) {
+	for(const char* const req : required) {
 		if (!cfg.has_attribute(req)) {
 			if(req == "create_map" && cfg.has_attribute("create_scenario")) {
 				// One of these is required, but not both
 				continue;
 			}
 			std::string msg = "Error when constructing a lua map generator -- missing a required attribute '";
-			msg += req + "'\n";
+			msg += req + std::string("'\n");
 			msg += "Config was '" + cfg.debug() + "'";
 			throw mapgen_exception(msg);
 		}