Add new authenticate method from gitblit

Gitblit seems to have added a further authenticate method, that was
still unimplemented in the plugin's UserService. We add a dummy
implementation for it.

Change-Id: I9c70778c366857391b9c9c671545abdbd1a13602

src/main/java/com/googlesource/gerrit/plugins/gitblit/auth/GerritToGitBlitUserService.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/gitblit/auth/GerritToGitBlitUserService.java b/src/main/java/com/googlesource/gerrit/plugins/gitblit/auth/GerritToGitBlitUserService.java
index a4bbb2c..ce74ac6 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/gitblit/auth/GerritToGitBlitUserService.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/gitblit/auth/GerritToGitBlitUserService.java
@@ -154,6 +154,11 @@
   }
 
   @Override
+  public UserModel authenticate(String username) {
+    return null;
+  }
+
+  @Override
   public UserModel authenticate(String username, SshKey key) {
     return null;
   }