PPP, ppp_init function is back, re-add magic_init

Work on PPP MEMPOOL re-added the previously removed ppp_init function
because we considered it useless. Re-add magic_init which was
previously in ppp_init and removed in commit 15fbfb7363, it does
not hurt doing so and might fill the gap if we failed understanding
the crypto implication about removing it.

src/netif/ppp/ppp.c

@@ -594,6 +594,12 @@ int ppp_init(void)
 
     LWIP_MEMPOOL_INIT(PPP_PCB);
 
+  /*
+   * Initialize magic number generator now so that protocols may
+   * use magic numbers in initialization.
+   */
+  magic_init();
+
     return 0;
 }