Allow compiling on Visual Studio 2012 and earlier.

Hi!

I have replaced the `#include <inttypes.h>` with `#include <stdint.h>` when compiling with Visual Studio 2012 and earlier. This is because Microsoft only added the inttypes.h header required for fully supporting C99 in Visual Studio 2013. This change is OK because mavlink doesn't seem to need inttypes.h but only stdint.h, which Visual Studio 2012 and earlier do have.

Thanks
Ryan