net: openthread: Fix build with NET_MGMT_EVENT_INFO disabled

In case NET_MGMT_EVENT module was enabled but w/o NET_MGMT_EVENT_INFO,
the OpenThread integration layer failed to build as the "info" field in
the net mgmt callback structure is not available then.

Fix this by conditionally enabling code processing the event only if
NET_MGMT_EVENT_INFO is enabled. Otherwise, print a warning, as the event
is not really useful if no address information is provided.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>