address.c: [RIOT] added stub for IN_MULTICAST

On platform RIOT, the IN_MULTICAST macro is missing. This change
therefore adds a stub to prevent build errors. (WIP)

src/address.c

@@ -27,6 +27,11 @@
 
 #include "address.h"
 
+#ifdef RIOT_VERSION
+/* FIXME */
+#define IN_MULTICAST(Address) (0)
+#endif /* RIOT_VERSION */
+
 int
 coap_address_equals(const coap_address_t *a, const coap_address_t *b) {
   assert(a); assert(b);