Usage of std::thread requires linking with appropriate system thread library.

This fixes the following link error (on debian sid, gcc 4.9):

Linking CXX executable pulseview
/usr/bin/ld: CMakeFiles/pulseview.dir/pv/sigsession.cpp.o: undefined reference to symbol 'pthread_create@@GLIBC_2.2.5'
//lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
CMakeFiles/pulseview.dir/build.make:2844: recipe for target 'pulseview' failed