Fixes for GCC 14 and libxml2 2.12.0 "error: Make more xmlError structs constant"

xml.c: In function ‘pg_xml_init’:
xml.c:1177:52: error: passing argument 2 of ‘xmlSetStructuredErrorFunc’ from incompatible pointer type [-Wincompatible-pointer-types]
 1177 |         xmlSetStructuredErrorFunc((void *) errcxt, xml_errorHandler);
      |                                                    ^~~~~~~~~~~~~~~~
      |                                                    |
      |                                                    void (*)(void *, xmlError *) {aka void (*)(void *, struct _xmlError *)}
In file included from /usr/include/libxml2/libxml/valid.h:15,
                 from /usr/include/libxml2/libxml/parser.h:19,
                 from xml.c:50:
/usr/include/libxml2/libxml/xmlerror.h:898:57: note: expected ‘xmlStructuredErrorFunc’ {aka ‘void (*)(void *, const struct _xmlError *)’} but argument is of type ‘void (*)(void *, xmlError *)’ {aka ‘void (*)(void *, struct _xmlError *)’}
  898 |                                  xmlStructuredErrorFunc handler);
      |                                  ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~

diff --git a/src/backend/utils/adt/xml.c b/src/backend/utils/adt/xml.c
index d3db75eb87..619f908c6d 100644
--- a/src/backend/utils/adt/xml.c
+++ b/src/backend/utils/adt/xml.c
@@ -124,7 +124,7 @@ static xmlParserInputPtr xmlPgEntityLoader(const char *URL, const char *ID,
 										   xmlParserCtxtPtr ctxt);
 static void xml_errsave(Node *escontext, PgXmlErrorContext *errcxt,
 						int sqlcode, const char *msg);
-static void xml_errorHandler(void *data, xmlErrorPtr error);
+static void xml_errorHandler(void *data, const xmlError *error);
 static int	errdetail_for_xml_code(int code);
 static void chopStringInfoNewlines(StringInfo str);
 static void appendStringInfoLineSeparator(StringInfo str);
@@ -2044,7 +2044,7 @@ xml_errsave(Node *escontext, PgXmlErrorContext *errcxt,
  * Error handler for libxml errors and warnings
  */
 static void
-xml_errorHandler(void *data, xmlErrorPtr error)
+xml_errorHandler(void *data, const xmlError *error)
 {
 	PgXmlErrorContext *xmlerrcxt = (PgXmlErrorContext *) data;
 	xmlParserCtxtPtr ctxt = (xmlParserCtxtPtr) error->ctxt;