PatchLineCommentsUtil: Add "final" to the public static PLC_ORDER.

This change was originally done with Iab765489d in the ChangeNotes
class on stable-2.11, but in the master branch the class has been
refactored out to PatchLineCommentsUtil.

Redo the change in the new location.

Change-Id: I4554658342b62a15b63589fa2c95d8a0fa27f2e8
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/PatchLineCommentsUtil.java b/gerrit-server/src/main/java/com/google/gerrit/server/PatchLineCommentsUtil.java
index 15519cc..7b182b1 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/PatchLineCommentsUtil.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/PatchLineCommentsUtil.java
@@ -68,7 +68,7 @@
  */
 @Singleton
 public class PatchLineCommentsUtil {
-  public static Ordering<PatchLineComment> PLC_ORDER =
+  public static final Ordering<PatchLineComment> PLC_ORDER =
       new Ordering<PatchLineComment>() {
     @Override
     public int compare(PatchLineComment c1, PatchLineComment c2) {