Fix `make depend` in sys/modules

[lwhsu]

Summary

Without this, make depend in sys/modules generates this error:

rm -f .depend
mkdep -f .depend -a   -nostdinc -D_KERNEL -DKLD_MODULE -I/usr/home/lwhsu/head/sys/modules/hyperv/netvsc/../../../dev/hyperv/netvsc -I. -I/usr/home/lwhsu/head/sys/modules/hyperv/netvsc/../../.. -std=iso9899:1999   /usr/home/lwhsu/head/sys/modules/hyperv/netvsc/../../../dev/hyperv/netvsc/hv_net_vsc.c /usr/home/lwhsu/head/sys/modules/hyperv/netvsc/../../../dev/hyperv/netvsc/hv_netvsc_drv_freebsd.c /usr/home/lwhsu/head/sys/modules/hyperv/netvsc/../../../dev/hyperv/netvsc/hv_rndis_filter.c
/usr/home/lwhsu/head/sys/modules/hyperv/netvsc/../../../dev/hyperv/netvsc/hv_netvsc_drv_freebsd.c:58:10: fatal error: 'opt_inet6.h' file not found
#include "opt_inet6.h"
         ^
1 error generated.
mkdep: compile failed
*** Error code 1

Test Plan

cd sys/modules; make depend

[lwhsu]

Committed as rS284969.