Merge #15968: Fix portability issue with pthreads

1b05dff Fix portability issue with pthreads (grim-trigger)

Pull request description:

  This change resolves the following issue:
  bitcoin/bitcoin#15951

  Only tested on OpenBSD 6.5/amd64

ACKs for commit 1b05df:
  fanquake:
    tACK 1b05dff. Tested on OpenBSD6.4 (`vagrant`).
  laanwj:
    utACK 1b05dff

Tree-SHA512: af48581af32820d5adc9ae5abb44f8f1b592c323f86fe2484108b81629389f6ef347598f9a087aa6476ac553e59828cd7927bb4ab11dc70e7c9a944a92fc54ae

Author: laanwj

src/util/threadnames.cpp

@@ -9,6 +9,11 @@
 #include <atomic>
 #include <thread>
 
+#if (defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__DragonFly__))
+#include <pthread.h>
+#include <pthread_np.h>
+#endif
+
 #include <util/threadnames.h>
 
 #ifdef HAVE_SYS_PRCTL_H