add missing {} in LifoSem

Reviewed By: magedm

Differential Revision: D33763949

fbshipit-source-id: 779c2a3d0337a357395a2554e810e7b0903a86d6

folly/synchronization/LifoSem.h

@@ -690,8 +690,9 @@ struct LifoSemBase {
           return head.idx();
         }
       } else {
-        if (skip_increment)
+        if (skip_increment) {
           return 0;
+        }
         auto after = head.withValueIncr(n);
         if (head_->compare_exchange_strong(head, after)) {
           // successful incr