include: add missing files to dist tarball

include: add missing files to dist tarball

When making a tarball (make dist) and trying to build from it, it fails
because of missing header files.  This patch adds all the necessary
headers to the dist tarball to make "./configure && make" from the
tarball succeed.

Change-Id: I89051cb060e911eac154ef1726034e905b49b085
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1243
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-by: Jonathan Rajotte Julien <jonathan.rajotte-julien@efficios.com>