net: dns: Fix the compilation even if DNS is disabled

Make sure that CONFIG_DNS_RESOLVER_MAX_SERVERS and
CONFIG_DNS_NUM_CONCUR_QUERIES are defined even if
CONFIG_DNS_RESOLVER is not enabled. This fixes compilation
error in this special case.

Change-Id: Icf1f247c138c379246ece57e78b04e70ae43cc1e
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>