Bug 21285: Add missing select2x2.png file

Bug 21285: Add missing select2x2.png file

This file is part of the Select2 jquery plugin, but wasn't added when
the use of the plugin was introduced in Koha.

To test:
- On a high DPI screen (retina mac?) on Chrome or Safari
- Edit an item
=> FAIL: dropdowns using Select2 (like 'Shelving location') don't display the icons (down arrow and cross) and a default value is wrongly selected
=> FAIL: the inspector says the requested select2x2.png file returned a 400 error.
- Apply this patch
- Reload the edit page
=> SUCCESS: Things display correctly! Dropdowns without selected values
by default!
=> SUCCESS: No errors fetching select2x2.png
- Sign off :-D

Signed-off-by: Jesse Maseto <jesse@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>