emulate/traffic: fix next usage

emulate/traffic: fix next usage

In commit b0c9de820c07d77c03b80505cb811ac1dac0808f, line 343:

    self.next_conversation_id = itertools.count().next

was changed to:

    self.next_conversation_id = next(itertools.count())

which is not correct, the first one is a function, the second one is a
int. This patch fixed it.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13573

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>