Cmake libxml fix

[fbergmann]

Description

This fixes the 2nd issue that alan came across, where the compile tests for libxml2 would fail for a static library, because of missing math symbols.

Motivation and Context

fixes #240

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Change in documentation

Checklist:

• I have updated all documentation necessary.
• I have checked spelling in (new) comments.

Testing

• Testing is done automatically and codecov shows test coverage
• This cannot be tested automatically