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

Author: aperezdc

Source/WebCore/xml/XSLTProcessor.h

@@ -61,7 +61,11 @@ class XSLTProcessor : public RefCounted<XSLTProcessor> {
 
     void reset();
 
+#if LIBXML_VERSION >= 21200
+    static void parseErrorFunc(void* userData, const xmlError*);
+#else
     static void parseErrorFunc(void* userData, xmlError*);
+#endif
     static void genericErrorFunc(void* userData, const char* msg, ...);
 
     // Only for libXSLT callbacks

Source/WebCore/xml/XSLTProcessorLibxslt.cpp

@@ -62,7 +62,11 @@ void XSLTProcessor::genericErrorFunc(void*, const char*, ...)
     // It would be nice to do something with this error message.
 }
 
+#if LIBXML_VERSION >= 21200
+void XSLTProcessor::parseErrorFunc(void* userData, const xmlError* error)
+#else
 void XSLTProcessor::parseErrorFunc(void* userData, xmlError* error)
+#endif
 {
     PageConsoleClient* console = static_cast<PageConsoleClient*>(userData);
     if (!console)