use u32_t instead of u_int32_t

The style u_int32_t is not used anywhere else in the project, and is not
supported by the C standard, now using lwIP u32_t type. It was introduced
in 25e398a.

src/netif/ppp/ipcp.c

@@ -972,7 +972,7 @@ static int ipcp_ackci(fsm *f, u_char *p, int len) {
 
 #define ACKCIWINS(opt, addr) \
     if (addr) { \
-	u_int32_t l; \
+	u32_t l; \
 	if ((len -= CILEN_ADDR) < 0) \
 	    goto bad; \
 	GETCHAR(citype, p); \