Clean up some includes

[peterbarker]

Most notable, don't include GCS_MAVLink/GCS.h everywhere. That header pulls a lot of stuff in - and there are only a handful of header files in the system that actually need anything more than the mavlink variable types (e.g. mavlink_message_t) out of it.

Several of the remaining uses of it I think show a flaw in the way we're doing our

#ifndef FOO_FEATURE_ENABLED`
#define FOO_FEATURE_ENABLED XYZZY
#endif

idiom. To get HAL_GCS_ENABLED those headers have to include the massive file and its many includes - probably most of the headers in the system, eventually.

We should consider either (a) moving to putting stuff in AP_HAL/AP_HAL_Boards.h, or doing something similar to AP_Filesystem/AP_Filesystem_Available - but with a better name.... perhaps GCS_MAVLink/config.h?

There's also (c), which is to stop guarding method definitions against the feature required. So e.g. AP_VisualOdom uses #if HAL_GCS_ENABLED whereas AP_OpticalFlow does not, despite them being roughly equivalent in terms of why they're interested in that define.

This has only been tested for compilation / autotest suite.