- ensure expat can be found when crosscompiling

[fbergmann]

Description

expat is currently not found when crosscompiling, this commit fixes the issue

Motivation and Context

fixes the issue that pyodide cant compile libsbml

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

[fbergmann]

@skeating, the test failure has nothing to do with the changes in this PR, just letting you know