Revert "DestinationFactory: remove @Singleton annotation"

This was done differently on master. Revert it so that the commit
from master can be backported instead.

This reverts commit dd7f52cdc70c5b08781ea4455e189839f1590349.

Change-Id: Id057d610b84d01ecf4c37b3a9547cf51bfe6e59f

src/main/java/com/googlesource/gerrit/plugins/replication/DestinationFactory.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/replication/DestinationFactory.java b/src/main/java/com/googlesource/gerrit/plugins/replication/DestinationFactory.java
index da7f162..09f4f4e 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/replication/DestinationFactory.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/replication/DestinationFactory.java
@@ -26,7 +26,9 @@
 import com.google.inject.Inject;
 import com.google.inject.Injector;
 import com.google.inject.Provider;
+import com.google.inject.Singleton;
 
+@Singleton
 public class DestinationFactory {
   private final Injector injector;
   private final PluginUser pluginUser;