Fix import error (#434)

Fixes:

$ nemo-preview
Cjs-Message: 13:54:44.506: JS WARNING: [/usr/share/nemo-preview/js/viewers/gst.js 34]: Requiring Gtk but it has 3 versions available; use imports.gi.versions to pick one

(nemo-preview-start:50721): Cjs-CRITICAL **: 13:54:44.507: JS ERROR: Error: Requiring Gtk, version none: Requiring namespace 'Gdk' version '4.0', but '3.0' is already loaded
@/usr/share/nemo-preview/js/viewers/gst.js:34:13

/usr/bin/nemo-preview: line 15: 50721 Segmentation fault      (core dumped) /usr/libexec/nemo-preview-start

nemo-preview/src/js/viewers/gst.js

@@ -26,6 +26,7 @@
  */
 
 imports.gi.versions.ClutterGst = '3.0';
+imports.gi.versions.Gtk = '3.0';
 imports.gi.versions.Gdk = '3.0';
 const ClutterGst = imports.gi.ClutterGst;
 const Clutter = imports.gi.Clutter;