ardupilot-solo headers can't be built with master

[Gussy]

The ardupilot-solo repo is using headers generated with upstream mavlink master. If master from this repo is used, the diff is large because of how upstream has changed comment styles and how various mavgen_c.py fixture outputs have changed.

TL;DR: This repo needs to be brought up to a more recent version of upsteam to avoid annoying dependency issues.