mlxsw: spectrum: compile-in dpipe support only if devlink is enabled

Makes no sense to have dpipe compiled in when devlink is not enabled,
because the devlink dpipe registation is noop function. So don't compile
it in. This also fixes missing extern structs errors.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: a86f0309 ("mlxsw: spectrum_dpipe: Add support for IPv4 host table dump")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>