added pthread to cmake

I got these errors:
```
/usr/bin/ld: ../libethereum/libethereum.a(PeerNetwork.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
```

and could resolve them by adding Threads (pthreads)