gre: add module alias for gre0 tunnel device

If gre is built as a module the 'ip tunnel add' command would fail because
the ip_gre module was not being autoloaded.  Adding an alias for
the gre0 device name cause dev_load() to autoload it when needed.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>