rgw: kill a compile warning for rgw_sync

killing the compile warning for

```
/ceph/src/rgw/rgw_sync.cc:1462:12:
warning: suggest explicit braces to avoid ambiguous ‘else’ [-Wparentheses]
         if (can_adjust_marker) yield {
            ^
```

Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>

src/rgw/rgw_sync.cc

@@ -1459,17 +1459,19 @@ class RGWMetaSyncShardCR : public RGWCoroutine {
 
       if (!lost_lock) {
         /* update marker to reflect we're done with full sync */
-        if (can_adjust_marker) yield {
-          sync_marker.state = rgw_meta_sync_marker::IncrementalSync;
-          sync_marker.marker = sync_marker.next_step_marker;
-          sync_marker.next_step_marker.clear();
-
-          RGWRados *store = sync_env->store;
-          ldout(sync_env->cct, 0) << *this << ": saving marker pos=" << sync_marker.marker << dendl;
-          using WriteMarkerCR = RGWSimpleRadosWriteCR<rgw_meta_sync_marker>;
-          call(new WriteMarkerCR(sync_env->async_rados, store, pool,
-                                 sync_env->shard_obj_name(shard_id),
-                                 sync_marker));
+        if (can_adjust_marker) {
+          yield {
+            sync_marker.state = rgw_meta_sync_marker::IncrementalSync;
+            sync_marker.marker = sync_marker.next_step_marker;
+            sync_marker.next_step_marker.clear();
+
+            RGWRados *store = sync_env->store;
+            ldout(sync_env->cct, 0) << *this << ": saving marker pos=" << sync_marker.marker << dendl;
+            using WriteMarkerCR = RGWSimpleRadosWriteCR<rgw_meta_sync_marker>;
+            call(new WriteMarkerCR(sync_env->async_rados, store, pool,
+                                   sync_env->shard_obj_name(shard_id),
+                                   sync_marker));
+          }
         }
         if (retcode < 0) {
           ldout(sync_env->cct, 0) << "ERROR: failed to set sync marker: retcode=" << retcode << dendl;