- helpers.cpp: add shortened strftime() function

taken from martiis neutrino-mp

src/system/helpers.h

@@ -54,6 +54,7 @@ std::string getFileName(std::string &file);
 std::string getFileExt(std::string &file);
 std::string getNowTimeStr(const char* format);
 std::string trim(std::string &str, const std::string &trimChars = " \n\r\t");
+std::string strftime(const char *format, const struct tm *tm);
 time_t toEpoch(std::string &date);
 std::string& str_replace(const std::string &search, const std::string &replace, std::string &text);
 std::string& htmlEntityDecode(std::string& text);