HID: Unexport struct uhid_hid_driver

As there are no external users this implementation detail does not need
to be exported.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: David Rheinsberg <david.rheinsberg@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>