LinuxKPI: net/if_inet6.h add struct inet6_dev { }

Add a dummy struct inet6_dev {}; to net/if_inet6.h. This is currently
not used for anything but in a declaration. Just needs to be there.

Sponsored by: The FreeBSD Foundation
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D30426

(cherry picked from commit ff09f9133f2fb80f705c2c742fc34291b05140af)