sys/random.h: add GRND_INSECURE from linux v5.6

added in

  linux commit 75551dbf112c992bc6c99a972990b3f272247e23
  random: add GRND_INSECURE to return best-effort non-cryptographic bytes

include/sys/random.h

@@ -10,6 +10,7 @@ extern "C" {
 
 #define GRND_NONBLOCK	0x0001
 #define GRND_RANDOM	0x0002
+#define GRND_INSECURE	0x0004
 
 ssize_t getrandom(void *, size_t, unsigned);