Fix linter warning.

Author: aardvark179

src/main/java/org/truffleruby/core/MarkingService.java

@@ -126,6 +126,7 @@ public void popStackPreservationFrame() {
     }
 
     public synchronized void runMarkersAndDropKeptList() {
+        @SuppressWarnings("unused")
         Object[] tmp = keptObjects;
         keptObjects = new Object[cacheSize];
         runAllMarkers();