Fix compilation on CentOS 6
Summary:
Issue:
processui/CMakeFiles/processui.dir/ProcessModel.cpp.o: In function `TimeUtil::systemUptimeSeconds()': ProcessModel.cpp:(.text._ZN8TimeUtil19systemUptimeSecondsEv[_ZN8TimeUtil19systemUptimeSecondsEv]+0x15): undefined reference to `clock_gettime'
Reviewers: jriddell, apol
Reviewed By: apol
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D5955