Silence GCC unused variable warning.

After making my previous commit to completely disable LWIP_ASSERT
macro invocations, I ended up with a warning in pbuf.c where an
err variable was set but only checked for success in an assert.  I
added a "(void)err;" reference to silence this warning.

libraries/net/lwip/lwip/core/pbuf.c

@@ -998,6 +998,8 @@ pbuf_coalesce(struct pbuf *p, pbuf_layer layer)
   }
   err = pbuf_copy(q, p);
   LWIP_ASSERT("pbuf_copy failed", err == ERR_OK);
+  /* next line references err variable even if LWIP_ASSERT is ignored. */
+  (void)err;
   pbuf_free(p);
   return q;
 }