net: dsa: tag_sja1105: Select CONFIG_PACKING

The packing facility is needed to decode Ethernet meta frames containing
source port and RX timestamping information.

The DSA driver selects CONFIG_PACKING, but the tagger did not, and since
taggers can be now compiled as modules independently from the drivers
themselves, this is an issue now, as CONFIG_PACKING is disabled by
default on all architectures.

Fixes: e53e18a6 ("net: dsa: sja1105: Receive and decode meta frames")
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: David S. Miller <davem@davemloft.net>