Move interface definition files to "inc" location

[chillfig]

Checklist (Please check before submitting)

• I reviewed the Contributing Guide.
• I performed a cursory search to see if the bug report is relevant, not redundant, nor in conflict with other tickets.

Is your feature request related to a problem? Please describe.
The interface definition files of all open source apps currently exist in the "src" directory.

Describe the solution you'd like
Create an "inc" dir to go with the "src" dir. Move the interface definitions into this location: "_msg.h", "_msgdefs.h", "_tbldefs.h", and "_events.h". Consider moving header files in both "platform_inc" and "mission_inc" to "inc"

Describe alternatives you've considered
Leaving as is.

Additional context
N/A

Requester Info
Justin Figueroa, Vantage Systems