Fix the following build error:
> [..]/xmlParser.cc:59:21: error: cannot initialize a variable of type 'xmlErrorPtr' (aka '_xmlError *') with an rvalue of type 'const xmlError *' (aka 'const _xmlError *')
> xmlErrorPtr error = xmlGetLastError();
> ^ ~~~~~~~~~~~~~~~~~
> [..]/xmlParser.cc:74:21: error: cannot initialize a variable of type 'xmlErrorPtr' (aka '_xmlError *') with an rvalue of type 'const xmlError *' (aka 'const _xmlError *')
> xmlErrorPtr error = xmlGetLastError();