Buck: Add missing manifest entry

Change-Id: Ice6bc31c1f2b98c72d64a090e45108463506ed02
diff --git a/BUCK b/BUCK
index f1b0c92..361ed93 100644
--- a/BUCK
+++ b/BUCK
@@ -1,4 +1,7 @@
 gerrit_plugin(
   name = 'singleusergroup',
   srcs = glob(['src/main/java/**/*.java']),
+  manifest_entries = [
+    'Gerrit-Module: com.googlesource.gerrit.plugins.singleusergroup.SingleUserGroup$Module',
+  ],
 )