mavgen_c.py: prefix header guards with ‘MAVLINK_’ to namespace them from...

... other application code

i already have a common.h in my project that uses COMMON_H as a header guard, so the mavlink common.h was not being included properly

Side note - I'm not actually sure how the permissions on that file got changed. Must have been the default behavior of one of my editors or something? If you'd like to introduce these edits as part of another commit that doesn't include the permissions changes, that would be totally fine by me!