Bug 27198: Fix syntax in marc21-retrieval-info-auth-dom.xml

This patch fixes the syntax in marc21-retrieval-info-auth-dom.xml,
so that one can use the Zebra special retrieval elements documented
at https://software.indexdata.com/zebra/doc/special-retrieval.html

These are very useful when troubleshooting issues with authority
records in Zebra.

Test plan (using KTD):
1. yaz-client unix:/var/run/koha/kohadev/authoritysocket
2. Enter these commands:
   . base authorities
   . format xml
   . elements zebra::snippet
   . find e
   . show 1
3. Result = diagnostic message:
     Diagnostic message(s) from database:
        [25] Specified element set name not valid for specified database
             -- v2 addinfo 'zebra::snippet'
4. Apply the patch
5. Copy updated file: sudo cp debian/templates/marc21-retrieval-info-auth-dom.xml
   /etc/koha/marc21-retrieval-info-auth-dom.xml
6. Stop and restart zebra (no need to rebuild the Zebra index):
   . sudo koha-zebra --stop kohadev
   . sudo pkill zebrasrv
   . sudo koha-zebra --start kohadev
7. Repeat commands in step 2.
8. Result from show 1 = XML output:
    Sent presentRequest (1+1).
    Records: 1
    Record type: XML
    <record xmlns="http://www.indexdata.com/zebra/">
      <snippet name="Any" type="w">Gontarski, S. <s>E</s></snippet>
    </record>nextResultSetPosition = 2
   Elapsed: 0.013929

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
(cherry picked from commit 10d5f6ef37)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>

debian/templates/marc21-retrieval-info-auth-dom.xml

@@ -14,6 +14,7 @@
             outputcharset="utf-8"/>
     </backend>
   </retrieval>
+  <retrieval syntax="xml"/>
   <retrieval syntax="xml" name="index"/> <!-- allow viewing index entries -->
   <retrieval syntax="xml" name="marc"
              identifier="info:srw/schema/1/marcxml-v1.1"/>