net: mscc: ocelot: select PACKING in the Kconfig

Ocelot now uses include/linux/dsa/ocelot.h which makes use of
CONFIG_PACKING to pack/unpack bits into the Injection/Extraction Frame
Headers. So it needs to explicitly select it, otherwise there might be
build errors due to the missing dependency.

Fixes: 40d3f295

 ("net: mscc: ocelot: use common tag parsing code with DSA")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>