.gitignore: ignore some vhost-user* related files

.gitignore: ignore some vhost-user* related files

Commit d52c454aadc creates
'/contrib/vhost-user-gpu/50-qemu-gpu.json' and '/vhost-user-gpu'
and commit 06914c97d3a creates '/vhost-user-input' neither of
which is ignored by git.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <db150a03315a89a849ab9968e4a5a26110d41424.1562942402.git.mprivozn@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>