Mklittlefs not listed in platform.json for 3.0.0 release

[earlephilhower]

The JSON for the 3.0.0 release does not contain an entry for mklittlefs in the toolsDependencies, meaning that file uploads no longer work through the IDE.

See reports in earlephilhower/arduino-esp8266littlefs-plugin#15 and the JSON file at https://arduino.esp8266.com/stable/package_esp8266com_index.json

[d-a-v]

That was in #6294 (package.json)