idl: gossip_digest: include required headers

To be self-sufficient, before the next patch
that will affect tagged_integer.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>

idl/gossip_digest.idl.hh

@@ -8,6 +8,9 @@
 
 #include "gms/inet_address_serializer.hh"
 #include "gms/version_generator.hh"
+#include "gms/generation-number.hh"
+
+#include "idl/utils.idl.hh"
 
 namespace gms {
 enum class application_state:int {