xsdconvert: xmlGetLastError(): fix invalid pointer conversion

This patch fixes the following errors observed with libxml2 v2.12.6:

XMLParser.cc: In static member function ‘static void XMLParser::warningHandler(void*, const char*, ...)’:
XMLParser.cc:193:38: error: invalid conversion from ‘const xmlError*’ {aka ‘const _xmlError*’} to ‘xmlErrorPtr’ {aka ‘_xmlError*’} [-fpermissive]
  193 |   xmlErrorPtr error = xmlGetLastError();
      |                       ~~~~~~~~~~~~~~~^~
      |                                      |
      |                                      const xmlError* {aka const _xmlError*}
XMLParser.cc: In static member function ‘static void XMLParser::errorHandler(void*, const char*, ...)’:
XMLParser.cc:213:38: error: invalid conversion from ‘const xmlError*’ {aka ‘const _xmlError*’} to ‘xmlErrorPtr’ {aka ‘_xmlError*’} [-fpermissive]
  213 |   xmlErrorPtr error = xmlGetLastError();
      |                       ~~~~~~~~~~~~~~~^~
      |                                      |
      |                                      const xmlError* {aka const _xmlError*}

Signed-off-by: Vadim Yanitskiy <vyanitskiy@sysmocom.de>

xsdconvert/XMLParser.cc

@@ -190,7 +190,7 @@ void XMLParser::warningHandler(void *, const char *, ...) {
     return;
   }
 
-  xmlErrorPtr error = xmlGetLastError();
+  const xmlError *error = xmlGetLastError();
 
   if (error->file == NULL) {
     fprintf(stderr,
@@ -210,7 +210,7 @@ void XMLParser::warningHandler(void *, const char *, ...) {
 }
 
 void XMLParser::errorHandler(void *, const char *, ...) {
-  xmlErrorPtr error = xmlGetLastError();
+  const xmlError *error = xmlGetLastError();
 
   if (error->code == XML_SCHEMAP_SRC_RESOLVE) {
     return;