packaging: exclude .gitkeep files

[hydrargyrum]

they're not useful in the package plus they look shady to some security tools

Description

just don't package the .gitkeep files which are useful for git but not for package

Related Issue

Motivation and Context

• generally cleaner though very minor
• avoids triggering security tools which (for instance) look for hidden files in unusual locations

How Has This Been Tested?

change is mostly cosmetic

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)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.