FreeBSD: add missing include of pthread_np.h

This is necessary for the pthread_attr_get_np function.

Change-Id: I01cfe075a7c86909e8cf37eb7f7c5d44fa044975
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2188310
Commit-Queue: Michaël Zasso <mic.besace@gmail.com>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Auto-Submit: Michaël Zasso <mic.besace@gmail.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67660}

src/base/platform/platform-freebsd.cc

@@ -6,6 +6,7 @@
 // parts, the implementation is in platform-posix.cc.
 
 #include <pthread.h>
+#include <pthread_np.h>
 #include <semaphore.h>
 #include <signal.h>
 #include <stdlib.h>