Fix linking with ld.lld

ld.lld apparently requires an explicit link to pthread for the
usage of std::call_once, otherwise it fails with:

ld: error: undefined symbol: pthread_once
>>> referenced by gthr-default.h:699 (/usr/include/c++/8.2.1/x86_64-pc-linux-gnu/bits/gthr-default.h:699)
>>>               CMakeFiles/kdevphplanguagesupport.dir/phpparsejob.cpp.o:(__gthread_once(int*, void (*)()))
collect2: error: ld returned 1 exit status