One more xmlsec include bug (seen with libxml2 2.12)
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This merge request is automatically cherrypicked from !1654 (merged) from source branch https://source.coderefinery.org/ellert/arc/-/tree/xmlsec-missing-libxml2-include orginally made by @ellert .
The original MR went to branch: https://source.coderefinery.org/nordugrid/arc/-/tree/next
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Build error on Fedora rawhide.