PPP, IAR EWARM won't compile ppp-new due to keyword clash

The word "class" is reserved in IAR's EWARM compiler since it looks like c++.
This causes a failure to compile in the lcp code.

Arguably it is a bug in the compiler, but it is easy to work around with a
name change in the lcp.[ch] code. I fixed it by changing "class" to "class_".

src/include/netif/ppp/lcp.h

@@ -97,7 +97,7 @@
 /* An endpoint discriminator, used with multilink. */
 #define MAX_ENDP_LEN	20	/* maximum length of discriminator value */
 struct epdisc {
-    unsigned char	class;
+    unsigned char	class_; /* -- The word "class" is reserved in C++. */
     unsigned char	length;
     unsigned char	value[MAX_ENDP_LEN];
 };