package/flatbuffers: bump to version 1.12.0

- Drop patch (already in version)
- Update hash of LICENSE.txt: Fixed Apache license not using canonical
  version:
  https://github.com/google/flatbuffers/commit/9834ee97874c95eee8b27677639eeb33aed12980


- Update indentation of hash file (two spaces)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>