src/Makefile.am: build {fmt} without `-Wmissing-noreturn`

Until <https://github.com/fmtlib/fmt/pull/3917> is merged and becomes
part of a release, let's just ignore missing `[[noreturn]]` warnings
when building {fmt}.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ia63f6248a59d8b42b35523e0a1972daa94f957bf
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12223
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>