Use nlohmann::json::object_t for dump entries

This is a similar change that was recently made for other entry types,
so also applying for dump entries.

I don't have a system that uses this resource, so I'm not able to test
this change.

Change-Id: I74ba2f05658c6c53d7e397e03aad31a44c7c3280
Signed-off-by: Jason M. Bills <jason.m.bills@intel.com>