Set visibility of ItsValidateComment's validCommit to private

The method is not used publicly, so we set it to private to minimize
the interface we have to keep stable.

Change-Id: I677c53eeb25792f911c0778cd2cc39ee06067315

hooks-its/src/main/java/com/googlesource/gerrit/plugins/hooks/validation/ItsValidateComment.java

diff --git a/hooks-its/src/main/java/com/googlesource/gerrit/plugins/hooks/validation/ItsValidateComment.java b/hooks-its/src/main/java/com/googlesource/gerrit/plugins/hooks/validation/ItsValidateComment.java
index fab325b..984e9e8 100644
--- a/hooks-its/src/main/java/com/googlesource/gerrit/plugins/hooks/validation/ItsValidateComment.java
+++ b/hooks-its/src/main/java/com/googlesource/gerrit/plugins/hooks/validation/ItsValidateComment.java
@@ -53,7 +53,7 @@
   @Inject
   private IssueExtractor issueExtractor;
 
-  public List<CommitValidationMessage> validCommit(ReceiveCommand cmd, RevCommit commit) throws CommitValidationException {
+  private List<CommitValidationMessage> validCommit(ReceiveCommand cmd, RevCommit commit) throws CommitValidationException {
     List<CommitValidationMessage> ret = Lists.newArrayList();
     ItsAssociationPolicy associationPolicy = getItsAssociationPolicy();