core/crypto: Add a HAS_RAND_BYTES constant

core/crypto/rand_bsd.odin

@@ -3,14 +3,13 @@ package crypto
 
 foreign import libc "system:c"
 
+HAS_RAND_BYTES :: true
+
 foreign libc {
 	arc4random_buf :: proc(buf: [^]byte, nbytes: uint) ---
 }
 
+@(private)
 _rand_bytes :: proc(dst: []byte) {
 	arc4random_buf(raw_data(dst), len(dst))
 }
-
-_has_rand_bytes :: proc() -> bool {
-	return true
-}