flow_dissector: remove unused function flow_get_hlen declaration

commit 56193d1b ("net: Add function for parsing the header length out
of linear ethernet frames") added this function declaration but it is
defined nowhere.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>