Fix a build failure on OpenBSD 4.x and MirBSD.

Fix a build failure on OpenBSD 4.x and MirBSD.

* sysdep.c (list_system_processes) [__OpenBSD__ || __MirBSD__]:
Make it a stub in this case; otherwise the build might fail,
and even if the build worked the function would just return nil anyway.
Problem reported by Nelson H. F. Beebe in
<http://lists.gnu.org/archive/html/emacs-devel/2013-03/msg00021.html>
and analyzed by Jérémie Courrèges-Anglas in
<http://lists.gnu.org/archive/html/emacs-devel/2013-03/msg00062.html>.

Fixes: debbugs:13881