json: use latest to get gcc10 fixes

nlohmann required some changes for gcc10. Those changes have not been
officially released yet by the project.

The following issues has details on the bug:
https://github.com/nlohmann/json/issues/1920

Change-Id: I853f36f961fcd094a3f399028182c308a970c2e5
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
1 file changed