Exclude gbp.conf from generated source packages.

gbp.conf is only useful while the source tree is a git working tree, as
soon as a source package is generated, it loses its usefulness. In
addition, excluding this, makes it possible to change gpb.conf (e.g., to
change the branch name for a stable-only upload) without this needlessly
cluttering the debdiff.

Git-Dch: Ignore