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/misc/meson.build

@@ -7,7 +7,7 @@ jsconf.set10('HAVE_BLUETOOTH', bt_dep.found())
 jsconf.set10('HAVE_NETWORKMANAGER', have_networkmanager)
 jsconf.set('datadir', datadir)
 jsconf.set('libexecdir', libexecdir)
-jsconf.set('nmprefixdir', nm_prefix)
+jsconf.set('vpndir', vpndir)
 
 config_js = configure_file(
   input: 'config.js.in',