ifconfig: Pacify a sign comparison warning in regdomain_sort.

Both ic_flags values are unsigned (uint32_t), so cast them to a signed
int to generate a signed result. Both ic_req values are also
unsigned, but since they are uint16_t, they are implicitly promited to
int before the subtraction.

Reported by: GCC -Wsign-compare
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D40610