updatehub-package-schema: Move serde_json to dev dependencies
updatehub-package-schema: Move serde_json to dev dependencies

The serde_json is used only on the tests.

Signed-off-by: Otavio Salvador <1d31b11c1b92bf5d043d36edfe7174db39a6b891@ossystems.com.br>

UpdateHub/updatehub