gcc9: quiet Waddress-of-packed-member for kernel build

This is lame, but it's what we already do for the clang build. We take
misaligned pointers into network header structures in many places.

Reviewed by: ian
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D22876

(cherry picked from commit 77acc3cfbcfbb2f66c683706e3b9e39d56c000fa)