[MLPL][StringUtils] Add a override function: toString

I want to use ""%"PRIu64" descriptor. But, I can't use it.
Because, When I use it, compiler outputs following error:

  CXX      StringUtils.lo
  StringUtils.cc: In static member function 'static std::string mlpl::StringUtils::toString(uint64_t)':
  StringUtils.cc:182:20: error: expected ')' before 'PRIu64' return sprintf("%"PRIu64, number);

  StringUtils.cc:182:34: warning: spurious trailing '%' in format [-Wformat=]
    return sprintf("%"PRIu64, number);

  StringUtils.cc:182:34: warning: too many arguments for format [-Wformat-extra-args]