virtgpu: Update kernel header

Generated using make headers_install.

This brings the C++ guard, proper include path for drm.h and the
switching to kernel types for fixed-with integers.

Generated from drm-next commit c11dea5b0290984fa48111957ba3fdc5b3bdae5a

Suggested-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Andreas Boll <andreas.boll.dev@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>