osdep: potentially fix compilation on OpenBSD

The <pthread_np.h> header expects that <pthread.h> was already included.
We were including <pthread.h> only later via our threads.h.

osdep/threads.c

@@ -16,6 +16,7 @@
  */
 #include <stdio.h>
 #include <errno.h>
+#include <pthread.h>
 
 #include "config.h"