linuxkpi: Add <linux/siphash.h>
The file is copied as is from Linux 6.10 as it dual-licensend under the
GPLv2 and BSD 3-clause.
The amdgpu DRM driver started to use it in Linux 6.10.
Reviewed by: bz, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54501