Don't include README.git in source packages

It doesn't make sense for non-git content anyways.

Change-Id: I9064de54657ab50caa526dab2830ee0d16054a9e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>