[SLIP]: trivial sparse warning fix

Function declared static in forward declaration, but not in actual code.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/slip.c b/drivers/net/slip.c
index 65bd20f..3fd4735 100644
--- a/drivers/net/slip.c
+++ b/drivers/net/slip.c
@@ -957,7 +957,7 @@
   *			STANDARD SLIP ENCAPSULATION		  	 *
   ************************************************************************/
 
-int
+static int
 slip_esc(unsigned char *s, unsigned char *d, int len)
 {
 	unsigned char *ptr = d;