[otns] fix compile issue when log region CORE is enabled (#6083)
[otns] fix compile issue when log region CORE is enabled (#6083)

This commit fixes an compile issue in otns.cpp when log region CORE is
enabled.
abtink/openthread,abtink/openthread,jwhui/openthread,abtink/openthread,abtink/openthread,jwhui/openthread,srickardti/openthread,openthread/openthread,jwhui/openthread,openthread/openthread,srickardti/openthread,srickardti/openthread,jwhui/openthread,openthread/openthread,openthread/openthread,srickardti/openthread