Cleanup weird syntax defining struct dns_ixfr

Cleanup weird syntax defining struct dns_ixfr

The struct dns_ixfr was defined as part of struct dns_xfrin, probably
because at some point it was an anonymous struct and then it was changed
to named struct with typedef at the top.  Move the definition from
struct dns_xfrin into and fold into the typedef ... dns_ixfr_t.