Bug 20193: Path to Greybox CSS broken after Bug 12904

Bug 20193: Path to Greybox CSS broken after Bug 12904

Bug 12904 changed the way CSS and JS assets should be linked, but the
Greybox CSS path wasn't changed in the process. This patch adds the
required VERSION template variable to greybox.inc.

To test, apply the patch and test usage of Greybox in the staff client.
For instance:

- Go to Tools -> Labels -> Manage -> Label batches
- Select a batch for export and click the 'Export selected' button
- A Greybox modal should be triggered, and the export process should
  proceed correctly.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>