If CONFIG_ENA_ETHERNET=y and CONFIG_DIMLIB=n,
below erros can be found:
drivers/net/ethernet/amazon/ena/ena_netdev.o: In function `ena_dim_work':
ena_netdev.c:(.text+0x21cc): undefined reference to `net_dim_get_rx_moderation'
ena_netdev.c:(.text+0x21cc): relocation truncated to
fit: R_AARCH64_CALL26 against undefined symbol `net_dim_get_rx_moderation'
drivers/net/ethernet/amazon/ena/ena_netdev.o: In function `ena_io_poll':
ena_netdev.c:(.text+0x7bd4): undefined reference to `net_dim'
ena_netdev.c:(.text+0x7bd4): relocation truncated to fit:
R_AARCH64_CALL26 against undefined symbol `net_dim'

After commit 282faf61 ("net: ena: switch to dim algorithm for rx adaptive
interrupt moderation"), it introduces dim algorithm, which configured by CONFIG_DIMLIB.
So, this patch is to select DIMLIB for ENA_ETHERNET.

Fixes: 282faf61

 ("net: ena: switch to dim algorithm for rx adaptive interrupt moderation")
Signed-off-by: Mao Wenan <maowenan@huawei.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>