ionic: add DIMLIB to Kconfig

>> ld.lld: error: undefined symbol: net_dim_get_rx_moderation
   >>> referenced by ionic_lif.c:52 (drivers/net/ethernet/pensando/ionic/ionic_lif.c:52)
   >>> net/ethernet/pensando/ionic/ionic_lif.o:(ionic_dim_work) in archive drivers/built-in.a

>> ld.lld: error: undefined symbol: net_dim
   >>> referenced by ionic_txrx.c:456 (drivers/net/ethernet/pensando/ionic/ionic_txrx.c:456)
   >>> net/ethernet/pensando/ionic/ionic_txrx.o:(ionic_dim_update) in archive drivers/built-in.a

v2: removed sketchy dashes in commit message

Fixes: 04a83459 ("ionic: dynamic interrupt moderation")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: David S. Miller <davem@davemloft.net>