pthread: Include pthread_np to comply with other platform.

[aokblast]

Summary

On NetBSD and Linux glibc, you only need to include pthread.h to use
pthread_{set,get}affinity_np. To sync with other implementation, we make
pthread_np.h be included in pthread.h to prevent some build system
failed on check features.

[aokblast]

Emm, it fails to compile on many places. I have to take a look at this.