crypto.rand: add a shim for `fn read(bytes_needed int) ?[]byte` in rand_default.c.v, so that it shows with `v doc`

vlib/crypto/rand/rand_default.c.v

@@ -0,0 +1,9 @@
+// Copyright (c) 2019-2021 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
+
+// read returns an array of `bytes_needed` random bytes read from the OS.
+pub fn read(bytes_needed int) ?[]byte {
+	return error('crypto.read is not implemented on this platform')
+}