util: Include pthread_np.h on FreeBSD

On FreeBSD pthread_set_name_np's prototype is provided by pthread_np.h.
As I believe it is the only platform to provide the "set_name" (with an
underscore) variant I hope it's fine to use the existing autoconf macro
HAVE_PTHREAD_SET_NAME_NP.

Signed-off-by: Ed Maste <emaste@freebsd.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>