s4-auth: added a dependency on com_err

s4-auth: added a dependency on com_err

this helps with the gentoo build. The problem is that without the
depenency, we don't add the cflags from the pkgconfig for com_err to
the build of auth/gensec. That really reflects a more general problem
with propogation of include dependencies, but this simple fix should
be enough for now.

Autobuild-User: Andrew Tridgell <tridge@samba.org>
Autobuild-Date: Sun Oct 31 13:13:33 UTC 2010 on sn-devel-104