8029141: Add @FunctionalInterface annotation to Callable interface

Reviewed-by: chegar, alanb

src/share/classes/java/util/concurrent/Callable.java

@@ -54,6 +54,7 @@ package java.util.concurrent;
  * @author Doug Lea
  * @param <V> the result type of method {@code call}
  */
+@FunctionalInterface
 public interface Callable<V> {
     /**
      * Computes a result, or throws an exception if unable to do so.