YARN-4816. Fix incompatible change in SystemClock.

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-common/src/main/java/org/apache/hadoop/yarn/util/SystemClock.java

@@ -38,11 +38,12 @@ public final class SystemClock implements Clock {
     return INSTANCE;
   }
 
-  private SystemClock() {
+  @Deprecated
+  public SystemClock() {
     // do nothing
   }
 
   public long getTime() {
     return System.currentTimeMillis();
   }
-}
+}