Git: ignore autogen, doxygen and gdb config files

Adds some additional files to .gitignore.

Change-Id: Ia5af298a91919fe00392282c0e5bb88fa2811a52
Reviewed-on: https://gerrit.libreoffice.org/8437
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>