crypto: fix rand module

vlib/crypto/rand/rand_darwin.c.v

@@ -0,0 +1,20 @@
+// Copyright (c) 2019-2020 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license
+// that can be found in the LICENSE file.
+
+module rand
+
+#include <Security/SecRandom.h>
+
+#flag darwin -framework Security
+
+fn C.SecRandomCopyBytes() int
+
+pub fn read(bytes_needed int) ?[]byte {
+	mut buffer := malloc(bytes_needed)
+	status := C.SecRandomCopyBytes(0, bytes_needed, buffer)
+	if status != 0 {
+		return read_error
+	}
+	return c_array_to_bytes_tmp(bytes_needed, buffer)
+}