Fix build with older glibc: link there to librt for clock_gettime
Summary:
Only since glibc version 2.17 -lrt is no longer needed
See same fix done for processui in bb556d8cea2fb77e66970ebc02303becd1c8e8e3
Reviewers: Plasma, davidedmundson, mart
Reviewed By: Plasma, davidedmundson, mart
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D24749