networkAgent: Pick up VPN service dir from pkg-config

It turns out that NetworkManager does export the directory as pkg-config
variable after all, so use that instead of building the path ourselves
from the prefix.

https://bugzilla.gnome.org/show_bug.cgi?id=789811

js/ui/components/networkAgent.js

@@ -594,7 +594,7 @@ var NetworkAgent = new Lang.Class({
         this._vpnRequests = { };
         this._notifications = { };
 
-        this._pluginDir = Gio.file_new_for_path(GLib.build_filenamev([Config.NMPREFIXDIR, 'lib/NetworkManager/VPN']));
+        this._pluginDir = Gio.file_new_for_path(Config.VPNDIR);
         try {
             let monitor = this._pluginDir.monitor(Gio.FileMonitorFlags.NONE, null);
             monitor.connect('changed', () => { this._vpnCacheBuilt = false; });