kmod: Fix building assym.o not building missing dependencies.

For instance, in the dtrace/dtrace module, building dtrace_asm.o wants
to build genassym.o first, but it doesn't build the missing ilinks
and if_*.h headers which are part of the OBJS_DEPEND_GUESS list
of dependencies to build if a .depend file is missing.

MFC after: 1 week
Sponsored by: Dell EMC Isilon