meson: link with -lrt if needed for clock_gettime

This is already done in autotools, and fixes the build with glibc < 2.17.

Signed-off-by: Loïc Yhuel <loic.yhuel@softathome.com>
3 jobs for meson-librt in 1 minute and 20 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Prep
passed #6331118
debian:buster@container-prep

00:00:11

 
  Build
passed #6331119
build-native-autotools

00:01:08

passed #6331120
build-native-meson

00:00:20