Enable automatic Build CI for code pushes and pull requests

[penguin359]

This enables a full CI build for each push to GitHub to validate the current state of the code on all branches and pull requests to master. This tests the build with multiple GCC versions, but I would like to add a few more. This is to replace the older Travis CI build system which is no longer provides for free to open source projects. This is configured to automatically trigger for PRs and show as part of the status to help quickly weed out build issues with them.

• Enabled CI build for GitHub Actions
• Added missing header to build
• Added matrix of GCC versions
• Limit matrix to known, good GCC versions

If you are curious about the missing header fix above, you can see the build failure due to it on this previous run:

https://github.com/penguin359/openlldp/actions/runs/9814786001/job/27102902583#step:10:427

[penguin359]

I recommend merging this request before accepting other PRs to detect possible build failures. Currently, PR #106 fails to build as shown by this GitHub Action.

[apconole]

I squashed all your github yaml changes into a single commit. If it's okay, I'll merge. The link to my branch: https://github.com/apconole/openlldp/tree/test_gha

[penguin359]

That's fine. It looks it's passing fully still so should be good to merge:

https://github.com/apconole/openlldp/actions/runs/10321316309

[apconole]

Merged. Thanks!