luci-mod-rpc: fix unresolved controller reference in rpc_auth()

Fixes the following error:

   .../rpc.lua:85: attempt to index global 'ctrl' (a nil value)

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-mod-rpc/luasrc/controller/rpc.lua

@@ -56,6 +56,7 @@ function index()
 end
 
 function rpc_auth()
+	local ctrl    = require "luci.controller.rpc"
 	local jsonrpc = require "luci.jsonrpc"
 	local http    = require "luci.http"
 	local sys     = require "luci.sys"