.gitignore: add some missing files

.gitignore: add some missing files

With this patch, doing:

    ./configure --enable-api-doc --enable-doxygen-doc --enable-man-pages --enable-python-bindings-doc --enable-python-bindings --enable-python-plugins
    make
    make dist

leaves a clean repo (no untracked files reported by `git status`).

Change-Id: Ifbe44eb0c93237dac933eeef3b39fabfa034601c
Signed-off-by: Erica Bugden <ebugden@efficios.com>
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13224
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>