[PPPOL2TP]: Label unused warning when CONFIG_PROC_FS is not set.

When CONFIG_PROC_FS is not set and CONFIG_PPPOL2TP is set,
we have the following warning in build:
  drivers/net/pppol2tp.c: In function 'pppol2tp_init':
  drivers/net/pppol2tp.c:2472: warning: label
'out_unregister_pppox_proto' defined but not used

This patches fixes this warning by adding appropriate #ifdef.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Acked-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>