Mark unused functions for cppcheck

We have a couple of functions that are unused (for now) by design.
Although at least one has a flag so that gcc doesn't warn, cppcheck has its
own warnings about this.  Add specific inline suppressions for these rather
than a blanket suppression in the Makefile.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

igmp.c

@@ -13,4 +13,5 @@
  */
 
 /* TO BE IMPLEMENTED */
+/* cppcheck-suppress unusedFunction */
 __attribute__((__unused__)) static void unused(void) { }