Add const to char * to quiet warnings.

Note two warnings in comments.  The serious one is that struct
interface->ifindex probably needs to be signed, ssince we store and
compare to -1.
diff --git a/zebra/kernel_socket.c b/zebra/kernel_socket.c
index e0f2a8f..9027cd6 100644
--- a/zebra/kernel_socket.c
+++ b/zebra/kernel_socket.c
@@ -276,6 +276,7 @@
 	      sdl = (struct sockaddr_dl *)cp;
 	      break;
             }
+	  /* XXX warning: pointer of type `void *' used in arithmetic */
 	  cp += SAROUNDUP(cp);
         }
     }
@@ -324,6 +325,10 @@
    * If ifp does not exist or has an invalid index (-1), create or
    * fill in an interface.
    */
+  /*
+   * XXX warning: comparison between signed and unsigned
+   * ifindex should probably be signed
+   */
   if ((ifp == NULL) || (ifp->ifindex == -1))
     {
       /*
@@ -812,7 +817,7 @@
 static void
 rtmsg_debug (struct rt_msghdr *rtm)
 {
-  char *type = "Unknown";
+  const char *type = "Unknown";
   struct message *mes;
 
   for (mes = rtm_type_str; mes->str; mes++)