make: add clean cmd

[Horki]

This Pull Request fixes/closes https://github.com/extrawurst/gitui/issues/1392 .

It changes the following:

• add clean to make file
• thats it

I followed the checklist:

• [ ] I added unittests
• [x] I ran make check without errors
• [x] I tested the overall application
• [ ] I added an appropriate item to the changelog