isdn: Fix missing ifdef in isdn_ppp

The following patch fixes a warning caused by a missing ifdef in
isdn_ppp.c. A function was defined, but never used if CONFIG_IPPP_FILTER
was not defined.

The warning was: 'get_filter' defined but not used
Patch is against 2.6.28.1

Signed-off-by: Daniele Venzano <venza@brownhat.org>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>