radvd: fix build against older glibc

clock_gettime() requires -lrt when linking against glibc older than 2.17.

Fixes:
http://autobuild.buildroot.net/results/e07/e07dd9a82ae4c8778b6901b9bd777f5affb9293a/



[Peter: add comment in .mk as well]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>