Make arc4random_buf declaration available on NetBSD

Signed-off-by: gufe44 <[email protected]>

ChangeLog.d/arc4random_buf-implicit.txt

@@ -0,0 +1,3 @@
+Bugfix
+   * Make arc4random_buf available on NetBSD and OpenBSD when _POSIX_C_SOURCE is
+     defined. Fix contributed in #3571. Adopted for LTS branch 2.16 in #3602.

tests/suites/main_test.function

@@ -25,6 +25,15 @@
 #endif
 #endif
 
+/*
+ * for arc4random_buf() from <stdlib.h>
+ */
+#if defined(__NetBSD__)
+#define _NETBSD_SOURCE 1
+#elif defined(__OpenBSD__)
+#define _BSD_SOURCE 1
+#endif
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include <mbedtls/config.h>
 #else