base-files: add "tty" user group

This is needed for an upcoming change to the hotplug default rules which
will cause /dev/tty* nodes to get assigned to the "tty" group in order
to support unprivileged user access when needed.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/base-files/files/etc/group

@@ -1,6 +1,7 @@
 root:x:0:
 daemon:x:1:
 adm:x:4:
+tty:x:5:
 mail:x:8:
 audio:x:29:
 www-data:x:33: