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

drivers/isdn/i4l/Kconfig

@@ -6,7 +6,7 @@ if ISDN_I4L
 
 config ISDN_PPP
 	bool "Support synchronous PPP"
-	depends on INET
+	depends on INET && NETDEVICES
 	select SLHC
 	help
 	  Over digital connections such as ISDN, there is no need to