drivers: isdn: Fix dependency for ISDN_PPP

Fix the following build warning:

warning: (ISDN_PPP) selects SLHC which has unmet direct dependencies (NETDEVICES)

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>