Adapt to change in Gerrit core

Change Id93e378be changed the signature of
MergeValidationListener#onPreMerge.

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: Ia8bfcbeeeff837c3dec6a9feefcfc3fd1e48e932
diff --git a/java/com/google/gerrit/plugins/checks/CheckerMergeValidator.java b/java/com/google/gerrit/plugins/checks/CheckerMergeValidator.java
index 1df2759..0e454eb 100644
--- a/java/com/google/gerrit/plugins/checks/CheckerMergeValidator.java
+++ b/java/com/google/gerrit/plugins/checks/CheckerMergeValidator.java
@@ -19,6 +19,7 @@
 import com.google.gerrit.server.IdentifiedUser;
 import com.google.gerrit.server.config.AllProjectsName;
 import com.google.gerrit.server.git.CodeReviewCommit;
+import com.google.gerrit.server.git.CodeReviewCommit.CodeReviewRevWalk;
 import com.google.gerrit.server.git.validators.MergeValidationException;
 import com.google.gerrit.server.git.validators.MergeValidationListener;
 import com.google.gerrit.server.project.ProjectState;
@@ -37,6 +38,7 @@
   @Override
   public void onPreMerge(
       Repository repo,
+      CodeReviewRevWalk revWalk,
       CodeReviewCommit commit,
       ProjectState destProject,
       BranchNameKey destBranch,