print: Avoid calling gdk_display_get_default

Check if GTK has been initialized before trying to get a display
in a class_init function. The introspection property dumper code
will instantiate all types and run into the new introduced errors
if we try to get a display in class_init.

gtk/print/gtkprintunixdialog.c

@@ -368,6 +368,9 @@ ensure_fallback_style (void)
   GdkDisplay *display;
   GtkCssProvider *provider;
 
+  if (!gtk_is_initialized ())
+    return;
+
   display = gdk_display_get_default ();
   if (!display)
     return;