Build fails with libxml2 version 2.12.0 due to API change

[aperezdc]

1bad176

Build fails with libxml2 version 2.12.0 due to API change
https://bugs.webkit.org/show_bug.cgi?id=265128

Reviewed by Philippe Normand.

Starting with libxml2 2.12.0, the API has changed the const-ness of the
xmlError pointers, which results in a build error due to a mismatched
type in the parsing error callback. This papers over the difference by
using preprocessor conditionals.

* Source/WebCore/xml/XSLTProcessor.h: Use const when building against
  libxml2 2.12.0 or newer.
* Source/WebCore/xml/XSLTProcessorLibxslt.cpp:
(WebCore::XSLTProcessor::parseErrorFunc): Ditto.

Canonical link: https://commits.webkit.org/270977@main

5677728

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🛠 gtk
	✅ 🧪 api-ios	🧪 mac-wk2	🧪 gtk-wk2
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv-sim
✅ 🛠 🧪 merge	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-early-warning-system]

EWS run on current version of this PR (hash 5677728)

Misc	iOS, tvOS & watchOS	macOS	Linux	Windows
✅ 🧪 style	✅ 🛠 ios	✅ 🛠 mac	✅ 🛠 wpe	✅ 🛠 wincairo
✅ 🧪 bindings	✅ 🛠 ios-sim	✅ 🛠 mac-AS-debug	🧪 wpe-wk2
✅ 🧪 webkitperl	✅ 🧪 ios-wk2	✅ 🧪 api-mac	✅ 🧪 api-wpe
	✅ 🧪 ios-wk2-wpt	✅ 🧪 mac-wk1	✅ 🛠 gtk
	✅ 🧪 api-ios	🧪 mac-wk2	🧪 gtk-wk2
	✅ 🛠 tv	✅ 🧪 mac-AS-debug-wk2	✅ 🧪 api-gtk
	✅ 🛠 tv-sim
✅ 🛠 🧪 merge	✅ 🛠 watch
	✅ 🛠 watch-sim

[webkit-commit-queue]

Committed 270977@main (1bad176): https://commits.webkit.org/270977@main

Reviewed commits have been landed. Closing PR #20741 and removing active labels.