gui: Link against pthread.

/usr/bin/ld: CMakeFiles/qapitrace.dir/apitracecall.cpp.o: undefined
reference to symbol '__pthread_key_create@@GLIBC_2.2.5'
/usr/bin/ld: note: '__pthread_key_create@@GLIBC_2.2.5' is defined in DSO
/lib64/libpthread.so.0 so try adding it to the linker command line
/lib64/libpthread.so.0: could not read symbols: Invalid operation

Signed-off-by: José Fonseca <jfonseca@vmware.com>