luci-lua-runtime: fix loading header/footer templates for SimpleForms

Fixes: #6054
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-lua-runtime/luasrc/dispatcher.lua

@@ -346,11 +346,11 @@ function invoke_form_action(model, ...)
 	end
 
 	http:header("X-CBI-State", state or 0)
-	tpl.render("header")
+	_G.L.include("header")
 	for i, res in ipairs(maps) do
 		res:render()
 	end
-	tpl.render("footer")
+	_G.L.include("footer")
 end