possibly missing -lrt in vdevd/helpers/LINUX/Makefile

[tilt12345678]

building vdev fails on amd64 libc 2.13 (wheezy) with
undefined reference to clock_gettime
proposed change: 9a49ede

[jcnelson]

Thanks for testing with Wheezy!

I'd be happy to merge the change. Want to send it as a PR so you'll get credit in the commit history?