include: Add struct declaration to ofp-print.h.

include: Add struct declaration to ofp-print.h.

If a libopenvswitch user includes ofp-print.h before ofp-util.h (which
is standard alphabetical order), and turns on -Werror, then they would
hit this compilation error in the include:

error: 'struct ofputil_port_map' declared inside parameter list will not
be visible outside of this definition or declaration [-Werror]
 void ofp_print(FILE *, const void *, size_t *, const struct ofputil_port_map *,

Fixes: 50f96b10e1c8 ("Support accepting and displaying port names in OVS tools.")
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>