PPP, moved ppp_init() from public API (ppp.h) to private API (ppp_impl.h)

ppp_init() is called by lwip_init(), users don't need to init PPP by themselves

src/include/netif/ppp/ppp.h

@@ -464,11 +464,6 @@ struct ppp_pcb_s {
  *** PUBLIC FUNCTIONS ***
  ************************/
 
-/*
- * Initialize the PPP subsystem.
- */
-int ppp_init(void);
-
 /*
  * Create a new PPP session.
  *

src/include/netif/ppp/ppp_impl.h

@@ -365,6 +365,9 @@ struct pppd_stats {
 
 /* PPP flow functions
  */
+/* initialize the PPP subsystem */
+int ppp_init(void);
+
 /* function called by pppoe.c */
 void ppp_input(ppp_pcb *pcb, struct pbuf *pb);