added -lrt option for building a MLPL's test.

This is a trial fix to resolve the following travis CI's errors.

$ ./server/mlpl/test/run-test.sh
../src/.libs/libmlpl.so: undefined reference to `sem_init'
../src/.libs/libmlpl.so: undefined reference to `pthread_mutex_trylock'
../src/.libs/libmlpl.so: undefined reference to `pthread_rwlock_rdlock'
../src/.libs/libmlpl.so: undefined reference to `sem_destroy'
../src/.libs/libmlpl.so: undefined reference to `pthread_rwlock_wrlock'
../src/.libs/libmlpl.so: undefined reference to `pthread_rwlock_destroy'
../src/.libs/libmlpl.so: undefined reference to `pthread_rwlock_init'
../src/.libs/libmlpl.so: undefined reference to `pthread_rwlock_unlock'
../src/.libs/libmlpl.so: undefined reference to `sem_post'
../src/.libs/libmlpl.so: undefined reference to `sem_wait'
collect2: ld returned 1 exit status
make[1]: *** [loggerTestee] Error 1
make: *** [all-recursive] Error 1