ppp: make ppp_auth.h self-contained

We need to include <stdint.h> to define 'uint8_t' and "ppp.h" for
'struct ppp_t'.
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>

accel-pppd/ppp/ppp_auth.h

 #ifndef PPP_AUTH_H
 #define PPP_AUTH_H
 
+#include <stdint.h>
+
 #include "list.h"
+#include "ppp.h"
 
 struct ppp_auth_handler_t;