[PPP]: Sparse warning fixes.

Fix a bunch of warnings in PPP and related drivers. Mostly because
sparse doesn't like it when the the function is only marked private in
the forward declaration.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/slhc.c

@@ -174,7 +174,7 @@ put16(unsigned char *cp, unsigned short x)
 
 
 /* Encode a number */
-unsigned char *
+static unsigned char *
 encode(unsigned char *cp, unsigned short n)
 {
 	if(n >= 256 || n == 0){
@@ -199,7 +199,7 @@ pull16(unsigned char **cpp)
 }
 
 /* Decode a number */
-long
+static long
 decode(unsigned char **cpp)
 {
 	register int x;