linuxkpi: Add <linux/siphash.h>

[dumbbell]

Summary

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.

This is part of the update of DRM drivers to Linux 6.10.

Sponsored by: The FreeBSD Foundation