[mlpl] Add size_t version of StringUtils::toString() to fix a build error

/home/aho/Projects/hatohol/server/mlpl/src/StringUtils.h:56:21: note: static std::string mlpl::StringUtils::t\
oString(int)
/home/aho/Projects/hatohol/server/mlpl/src/StringUtils.h:57:21: note: static std::string mlpl::StringUtils::t\
oString(uint64_t)
In file included from Helpers.h:30:0,
                 from AssertJoin.h:23,
                 from testSQLProcessor.cc:37: