resolv/tst-resolv-noaaaa: Support building for older C standards

This avoids a compilation error:

tst-resolv-noaaaa.c: In function 'response':
tst-resolv-noaaaa.c:74:11: error: a label can only be part of a statement and a declaration is not a statement
           char ipv4[4] = {192, 0, 2, i + 1};
           ^~~~
tst-resolv-noaaaa.c:79:11: error: a label can only be part of a statement and a declaration is not a statement
           char *name = xasprintf ("ptr-%d", i);
           ^~~~