Add a hint that "device ctl" kind of depends on "device iscsi".

It actually doesn't - "device ctl" automatically pulls in ICL, which
would normally be a part of iscsi.ko. However, doing it that way makes
iscsi.ko unloadable, and building ctl.ko without iscsi.ko (using
MODULES_OVERRIDE) results in ctl.ko that is unloadable, due to missing
symbols that would be resolved to iscsi.ko. And since the symbols
are named "icl_whatever", it's not obvious that it's iscsi.ko that's
required.

If there is a better way - let me know.

MFC after: 1 month
Sponsored by: The FreeBSD Foundation