Point gitiles submodule to gitiles security fix repo

This makes it so that we can release and test security fixes
in the gitiles plugin.

After this commit, git has changed the URL of the submodule:
Submodule 'plugins/gitiles' (sso://gerrit/plugins/gitiles-security-fixes) registered for path 'plugins/gitiles'
Submodule 'plugins/hooks' (sso://gerrit/plugins/hooks) registered for path 'plugins/hooks'
Submodule 'plugins/plugin-manager' (sso://gerrit/plugins/plugin-manager) registered for path 'plugins/plugin-manager'
Submodule 'plugins/replication' (sso://gerrit/plugins/replication) registered for path 'plugins/replication'
Submodule 'plugins/reviewnotes' (sso://gerrit/plugins/reviewnotes) registered for path 'plugins/reviewnotes'
Submodule 'plugins/singleusergroup' (sso://gerrit/plugins/singleusergroup) registered for path 'plugins/singleusergroup'
Submodule 'plugins/webhooks' (sso://gerrit/plugins/webhooks) registered for path 'plugins/webhooks'

Change-Id: Id340f5932b663bdd309a0dc69f35a679103a4274
diff --git a/.gitmodules b/.gitmodules
index 9f67e77..2e39ff2 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -24,7 +24,7 @@
 
 [submodule "plugins/gitiles"]
 	path = plugins/gitiles
-	url = ../plugins/gitiles
+	url = ../plugins/gitiles-security-fixes
 	branch = .
 
 [submodule "plugins/hooks"]