[SLIP]: trivial sparse warning fix

Function declared static in forward declaration, but not in actual code.
Signed-off-by: NStephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/slip.c

@@ -957,7 +957,7 @@ slip_close(struct tty_struct *tty)
   *			STANDARD SLIP ENCAPSULATION		  	 *
   ************************************************************************/
 
-int
+static int
 slip_esc(unsigned char *s, unsigned char *d, int len)
 {
 	unsigned char *ptr = d;