fix: int main for gregorian_time.c prorgram

[hkershaw-brown]

Description:

Add int to main for gregorian_time.c program for compilers that enforce ISO C99

Fixes issue

Fixes #961

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 not work as expected)
• Documentation update

Documentation changes needed?

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.

Tests

Please describe any tests you ran to verify your changes.
Compiled, ran, checked the return code from a fail and and a success

> icx gregorian_time.c 
> ./a.out 123456 40
123456    40 == 1939/01/06 00:00:40
> echo $?
0
> ./a.out -123456 0 
error: days must be non-negative
> echo $?
254

Checklist for merging

• Updated changelog entry
• Documentation updated
• Update conf.py

Checklist for release

• Merge into main
• Create release from the main branch with appropriate tag
• Delete feature-branch

Testing Datasets

• Dataset needed for testing available upon request
• Dataset download instructions included
• No dataset needed

[Copilot]

Pull Request Overview

This PR fixes a C compilation issue by adding the missing int return type to the main function in gregorian_time.c. This change ensures compliance with ISO C99 standards, as some compilers enforce explicit return type declarations for all functions, including main.

• Added int return type to the main function declaration

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[mjs2369]

Ready to merge