.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 <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <db150a03315a89a849ab9968e4a5a26110d41424.1562942402[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>