resolv: Fix building tst-resolv-invalid-cname for earlier C standards

This fixes this compiler error:

tst-resolv-invalid-cname.c: In function ‘test_mode_to_string’:
tst-resolv-invalid-cname.c:164:10: error: label at end of compound statement
     case test_mode_num:
          ^~~~~~~~~~~~~

Fixes commit 9caf782
("resolv: Add new tst-resolv-invalid-cname").

resolv/tst-resolv-invalid-cname.c

@@ -162,7 +162,7 @@ test_mode_to_string (enum test_mode mode)
     case gai_canon:
       return "gai_canon";
     case test_mode_num:
-      /* Report error below.  */
+      break;                    /* Report error below.  */
     }
   FAIL_EXIT1 ("invalid test_mode: %d", mode);
 }