repair windows builds (2.4)

repair windows builds (2.4)

commit 0c1cc8d65539f removed "DWORD len;" from service_enable_dhcp(),
but that variable is far from "unused" - breaking compilation.

The problem here was that it's a larger "cleanup compiler warnings"
patch which was fully correct for master, but this particular function
looks different in release/2.4 because the whole iservice call
refactoring patch isn't in release/2.4 - so master does not need the
DWORD len, but release/2.4 does.

More care needed with "warning cleanup"...

Signed-off-by: Gert Doering <gert@greenie.muc.de>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20190705130001.30741-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18652.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>