LinuxKPI: add dummy header file linux/unaligned.h

Long-term asm/unaligned.h is likely to migrate to this file?

Sponsored by: The FreeBSD Foundation
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D50003

(cherry picked from commit 28efbf9d2f67ce0bb24ad83462c99a1c2771e602)