flow: Make flow_table.h #include the protocol specific headers it needs

flow_table.h, the lower level flow header relies on having the struct
definitions for every protocol specific flow type - so far that means
tcp_conn.h.  It doesn't include it itself, so tcp_conn.h must be included
before flow_table.h.

That's ok for now, but as we use the flow table for more things,
flow_table.h will need the structs for all of them, which means the
protocol specific .c files would need to include tcp_conn.h _and_ the
equivalents for every other flow type before flow_table.h every time,
which is weird.

So, although we *mostly* lean towards the include style where .c files need
to handle the include dependencies, in this case it makes more sense to
have flow_table.h include all the protocol specific headers it needs.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

flow.c

@@ -15,7 +15,6 @@
 #include "siphash.h"
 #include "inany.h"
 #include "flow.h"
-#include "tcp_conn.h"
 #include "flow_table.h"
 
 const char *flow_type_str[] = {