Bug 1650323 - Add suppress "unchecked" for a TreeBuilder method. DONTBUILD NPOTB r=alchen

In `nu.validator.htmlparser.impl.TreeBuilder`, this change adds
`@SuppressWarnings("unchecked")` to the `getUnusedStackNode()` method.

Differential Revision: https://phabricator.services.mozilla.com/D82149

parser/html/javasrc/TreeBuilder.java

@@ -4787,7 +4787,7 @@ public abstract class TreeBuilder<T> implements TokenHandler,
         }
     }
 
-    private StackNode<T> getUnusedStackNode() {
+    @SuppressWarnings("unchecked") private StackNode<T> getUnusedStackNode() {
         // Search for an unused stack node.
         while (stackNodesIdx < numStackNodes) {
             if (stackNodes[stackNodesIdx].isUnused()) {