[cluster] Fix non-mpi compile

fix compile of the cluster branch in the non-mpi case.

Add a TODO as a reminder for the new voting scheme.

No functional changes

src/cluster.cpp

@@ -232,6 +232,8 @@ void save(Thread* thread, TTEntry* tte,
   }
 }
 
+
+// TODO update to the scheme in master.. can this use aggregation of votes?
 void pick_moves(MoveInfo& mi) {
 
   MoveInfo* pMoveInfo = NULL;

src/cluster.h

@@ -95,7 +95,7 @@ inline void save(Thread* thread, TTEntry* tte, Key k, Value v, Bound b, Depth d,
   tte->save(k, v, b, d, m, ev);
 }
 inline void pick_moves(MoveInfo&) { }
-void sum(uint64_t& val) { }
+inline void sum(uint64_t& val) { (void)val; }
 inline void sync_start() { }
 inline void sync_stop() { }