iavf(4): Include RSS header file when RSS is defined

This should unbreak the kernel build when "options RSS" is
defined in the kernel configuration, and make the feature work.

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

Reported by: adrian@
Sponsored by: Intel Corporation

(cherry picked from commit 5577aa338a675f2a465a58356894fbe24ba20407)