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 9caf7822
("resolv: Add new tst-resolv-invalid-cname").

(cherry picked from commit d09aa4a1)