linuxkpi: Add <linux/font.h>

The header defines the minimum to allow the DRM generic code to build.
The only function used so far is get_default_font(), as part of a
DRM-specific panic handler. We do not use this panic handler on FreeBSD.

For now, it returns a NULL value. It should probably map to a vt(4)
font.

The DRM generic code started to use it in Linux 6.10 as part of the
panic handler.

Reviewed by: bz, christos
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54491