[mlpl] Add -lrt linker flag.

This is a trial to fix the following error on TravisCI.

$ ./server/mlpl/test/run-test.sh
../src/.libs/libmlpl.so: undefined reference to `clock_gettime'
collect2: ld returned 1 exit status
make[1]: *** [loggerTestee] Error 1
make: *** [all-recursive] Error 1