networkAgent: Update VPN config path

The location moved a while ago, so update the path to point to
the non-deprecated location.

https://bugzilla.gnome.org/show_bug.cgi?id=791487

js/misc/config.js.in

@@ -14,6 +14,6 @@ var GETTEXT_PACKAGE = '@GETTEXT_PACKAGE@';
 var LOCALEDIR = '@datadir@/locale';
 /* other standard directories */
 var LIBEXECDIR = '@libexecdir@';
-var SYSCONFDIR = '@sysconfdir@';
+var NMPREFIXDIR = '@nmprefixdir@';
 /* g-i package versions */
 var LIBMUTTER_API_VERSION = '@LIBMUTTER_API_VERSION@'