config/zfs-build.m4: add Gentoo's bash-completion path

Followup e69ade32e116e72d03068c03799924c3f1a15c95 by adding Gentoo's
bash completion path.

We should probably consider using/honouring the standard --with-bashcompletiondir
autoconf option as well, but that's something to do later.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Umer Saleem <usaleem@ixsystems.com>
Signed-off-by: Sam James <sam@gentoo.org>
Closes #15372