output-flow.h: include flow.h and decode.h

output-flow.h: include flow.h and decode.h

A library/plugin user wanting to register a custom flow logger must
include "output-flow.h", however that depends on some other includes.
One train of thought with respect to include files in libraries, is
that they should include all their dependencies on behalf of the
user. To make a custom flow logger just a little easier, include
"flow.h" and "decode.h".

Ticket: #7227