deskutils/recoll: unbreak with new libxml2 API

which broke because the newer libxml2 error reporting callbacks
now want functions that take const-qualified pointers.

PR: 279960
Approved by: danfe@ (maintainer timeout, 38 days)
Approved by: portmgr@ (just-fix-it blanket approval)
MFH: 2025Q3

Not bumping PORTREVISION this only fixes the build,
and we have one more fix coming up.