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
Merge pull request #242 from liamstask/header-namespace

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