headers/bsd: Export pthread_attr_get_np

Exported `pthread_attr_get_np` under `_DEFAULT_SOURCE`. Otherwise,
the presence of this function in `libroot.so` without a definition
would confuse some configuration scripts.

Change-Id: I623ebefc98800e9528dc48859c2aefc828821f27
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6383
Reviewed-by: Jessica Hamilton <jessica.l.hamilton@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>

headers/compatibility/bsd/pthread.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright 2023 Haiku, Inc. All Rights Reserved.
+ * Distributed under the terms of the MIT License.
+ */
+#ifndef _BSD_PTHREAD_H_
+#define _BSD_PTHREAD_H_
+
+
+#include_next <pthread.h>
+#include <features.h>
+
+
+#ifdef _DEFAULT_SOURCE
+
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+extern int pthread_attr_get_np(pthread_t thread, pthread_attr_t* attr);
+
+
+#ifdef __cplusplus
+}
+#endif
+
+
+#endif
+
+
+#endif	/* _BSD_PTHREAD_H_ */
+