fileUtils: Fix fallback code for old gjs

gjs's D-Bus convenience explicitly expects a string representation
of an interface, but the new convenience method to load an XML
description from a resource introduced in commit f42d9df3e0 only
returns a string when using gjs from the GNOME 3.30 release. We
have so far managed to keep compatibility with the previous stable
gjs release, so fix up the fallback code to cast to string.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/578

js/misc/fileUtils.js

@@ -90,7 +90,7 @@ function loadInterfaceXML(iface) {
         if (bytes instanceof Uint8Array)
             xml = imports.byteArray.toString(bytes)
         else
-            xml = bytes;
+            xml = bytes.toString();
     } catch (e) {
         log('Failed to load D-Bus interface ' + iface);
     }