src.ctf.lttng-live: add IWYU pragma

src.ctf.lttng-live: add IWYU pragma

clangd reports that the include of cpp-common/bt2c/fmt.hpp is unused,
but it is in fact used for the formatting of some types.  Add a pragma.

Change-Id: I850fb44383b61bd46880c4eb67bd9a2323c0d725
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/13253
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>