netlink.h: s/typeof/__typeof

typeof() does not exist in -std=c99 mode and the relevant #define is
only for _KERNEL, so use __typeof here instead.

Reviewed by: jhb
MFC after: 2 weeks
Pull Request: https://github.com/freebsd/freebsd-src/pull/1070

(cherry picked from commit 4261507a5e3e73ef6f9535935d22785056954b19)