Bug 21058: Added HTML class to showRDAtag264

Bug 21058: Added HTML class to showRDAtag264

Adds the HTML class "rda264" to the span generated by the XSLT function
showRDAtag264.

To test, same as previous, using a notice with a 264 tag. Both patches
only apply to MARC21.

I was unsure how to name the class, so I've chose the simplest, most
explicit name.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 072a901d35844e99db65c1069b52f4f8b5ad6e5a)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>