add a minimal .gitignore back

It was entirely deleted along with autotools, but adding this simple one
will cover most people's needs.

Suggested-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>