genassym: Remove stale *if.h depends

Since we no longer need to include intr.h, which depends on sys/bus.h,
in genassym, we no longer need to include it in the Makefile depends.

PR: 283041
Sponsored by: Netflix
Reviewed by: ehem_freebsd_m5p.com, mmel, andrew
Differential Revision: https://reviews.freebsd.org/D47848