make gcc shut up

If -flto is used then gcc will generate a lot more warnings than before,
among them a number of use-without-initialization warnings. Most of them
without are false positives, but let's make them go away, because it
doesn't really matter.

src/nspawn/nspawn.c

@@ -743,7 +743,7 @@ static int setup_resolv_conf(const char *dest) {
 
 static int setup_boot_id(const char *dest) {
         _cleanup_free_ char *from = NULL, *to = NULL;
-        sd_id128_t rnd;
+        sd_id128_t rnd = {};
         char as_uuid[37];
         int r;