[concurrency] Provide missing header file. (#37004)

HAVE_PTHREAD_H is supplied in this config file.

The `__has_include` pattern is discouraged, but there is no other way of
conditioning on `pthread_np.h`, which is required for `pthread_main_np`
on OpenBSD. Therefore, use it, but call it out in a comment.

(cherry picked from commit eb945c8)

Author: 3405691582

stdlib/public/Concurrency/Actor.cpp

@@ -34,6 +34,7 @@
 #include "swift/Runtime/ThreadLocalStorage.h"
 #include "swift/ABI/Task.h"
 #include "swift/ABI/Actor.h"
+#include "llvm/Config/config.h"
 #include "llvm/ADT/PointerIntPair.h"
 #include "TaskPrivate.h"
 #include "VoucherSupport.h"
@@ -61,6 +62,11 @@
 
 #if HAVE_PTHREAD_H
 #include <pthread.h>
+
+// Only use __has_include since HAVE_PTHREAD_NP_H is not provided.
+#if __has_include(<pthread_np.h>)
+#include <pthread_np.h>
+#endif
 #endif
 
 #if defined(_WIN32)