make the rebuilding boolean volatile

Author: Vladimir Kotal

suggester/src/main/java/org/opengrok/suggest/Suggester.java

@@ -91,7 +91,7 @@ public final class Suggester implements Closeable {
 
     private final int rebuildParallelismLevel;
 
-    private boolean rebuilding;
+    private volatile boolean rebuilding;
     private final Lock rebuildLock = new ReentrantLock();
     private final Condition rebuildDone = rebuildLock.newCondition();