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

This also avoids multiple evaluations in STRV_FOREACH_BACKWARDS()

src/test/test-string-util.c

@@ -865,7 +865,6 @@ TEST(strverscmp_improved) {
                 "124",
                 NULL,
         };
-        const char * const *p, * const *q;
 
         STRV_FOREACH(p, versions)
                 STRV_FOREACH(q, p + 1)