Bring back removed AUX_DIAG in Kconfig
AUX_DIAG is needed to be able to enable SNMP_AUX_DIAG.
AUX_DIAG was removed in the commit:
df529bbb Kconfig, configs: remove AUX_DIAG (unused)
AUX_DIAG is needed to be able to enable SNMP_AUX_DIAG.
AUX_DIAG was removed in the commit:
df529bbb Kconfig, configs: remove AUX_DIAG (unused)