Added missing <cstdint> include in html2md

[vincent-noel]

Description

Added a missing cstdint include in src/sbml/html2md/html2md.h

Motivation and Context

Without this import, when compiling with GCC 15, an error is raised as reported in #439

Types of changes

• [X] 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
• [X] This cannot be tested automatically

This needs to be tested with GCC15. I tested it on MSYS2 and ubuntu 25.