strv: make iterator in STRV_FOREACH() declaread in the loop

This also avoids multiple evaluations in STRV_FOREACH_BACKWARDS()

src/nspawn/nspawn.c

@@ -3149,7 +3149,6 @@ static int patch_sysctl(void) {
         };
 
         unsigned long flags;
-        char **k, **v;
         int r;
 
         flags = effective_clone_ns_flags();