Fix incompatible type conversion errors

A recent libxml2 changed its handler function to take a `const` pointer. It's
safe for us to have it, and them not, but the opposite way causes an implicit
cast warning.

This fixes the build in Fedora 40, which changes several incompatible pointer
conversion warnings into errors.

This is a forward port of /r/2944/

Testing Done:
Compiled on Fedora Rawhide.

Reviewed at https://reviews.imfreedom.org/r/2950/