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