samba: add winbind.service to winbind package

This solves the following warning:

lib32-samba-3.6.24: lib32-samba: Files/directories were installed but not shipped
  /lib/systemd/system/winbind.service [installed-vs-shipped]

Signed-off-by: Chong Lu <Chong.Lu@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>