Fix incorrect formatting

Gets rid of this warning:

  error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]

src/GameCommand.cpp

@@ -738,7 +738,7 @@ void GameCommand::ApplySelf( const vector<PlayerNumber> &vpns ) const
 			}
 			break;
 		default:
-			LuaHelpers::ReportScriptError("Invalid StyleType: " + m_pStyle->m_StyleType);
+			LuaHelpers::ReportScriptErrorFmt("Invalid StyleType: %d", m_pStyle->m_StyleType);
 		}
 	}
 	if( m_dc != Difficulty_Invalid )