Fix LINT-NOINET kernels

• Generate opt_inet.h appropriately according to MK_INET_SUPPORT
• Only add in_gif.c to SRCS if MK_INET_SUPPORT != no

This is a surgical fix since the build infrastructure has diverged sufficiently
between stable/10 and head.

This change will likely be reverted if and when imp's changes to config.mk have
been backported to ^/stable/10 .

Reviewed by: ae (no serious objections)
Differential Revision: https://reviews.freebsd.org/D4864
MFC after: 1 week
Sponsored by: EMCC / Isilon Storage Division