automake: Include virtgpu_drm.h in the release tarball

The plan is to use this version of virtgpu_drm.h in mesa and drop mesa's
local copy.
To actually use this header it needs to be shipped in the tarball.

This was missed in c745e541

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