use flexible arrays

[roblatham00]

newer gcc-11 will complain about overrunning arrays if we don't use the
C99 standard "flexible array" notation

Description

the 'tinyexpr' library used an old school "grow this last member of a struct" approach. gcc-11 flags that as an array overrun.

Motivation and Context

Since Unify build with Werror (we can talk more about that later...), when I build with gcc-11 the array overrun warnings cause the build to fail

How Has This Been Tested?

Tested on my laptop.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Testing (addition of new tests or update to current tests)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the UnifyFS code style requirements.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted.

[MichaelBrim]

Thanks Rob.