fixed the name of LibXml2 in CMakeLists.txt
fixed the name of LibXml2 in CMakeLists.txt

The case of "LibXML2" in CMakeLists.txt is not the same as specified in the "FindLibXml2" CMake module.
This causes CMake  unable to find libxml2 on case-sensitive file systems such as ext4.

xcsp3team/XCSP3-CPP-Parser,pothitos/XCSP3-CPP-Parser,xcsp3team/XCSP3-CPP-Parser