PPP: fix constness in PPP related files when PRINTPKT_SUPPORT is enabled

src/include/netif/ppp/mppe.h

@@ -101,7 +101,7 @@
 /* The reverse of the above */
 #define MPPE_CI_TO_OPTS(ci, opts)		\
     do {					\
-	u_char *ptr = ci; /* u_char[4] */	\
+	const u_char *ptr = ci; /* u_char[4] */	\
 						\
 	opts = 0;				\
 						\

src/include/netif/ppp/ppp_impl.h

@@ -290,7 +290,7 @@ struct protent {
     void (*close) (ppp_pcb *pcb, const char *reason);
 #if PRINTPKT_SUPPORT
     /* Print a packet in readable form */
-    int  (*printpkt) (u_char *pkt, int len,
+    int  (*printpkt) (const u_char *pkt, int len,
 			  void (*printer) (void *, const char *, ...),
 			  void *arg);
 #endif /* PRINTPKT_SUPPORT */