GCS_MAVLink: rename OpticalFlow class to AP_OpticalFlow

Brings us in-line with other classes in ArduPilot.

Removes ambiguity with AP_HAL::OpticalFlow which can cause compilation errors as we start to make code more portable across targets

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -2414,7 +2414,7 @@ MAV_RESULT GCS_MAVLINK::_set_mode_common(const MAV_MODE _base_mode, const uint32
  */
 void GCS_MAVLINK::send_opticalflow()
 {
-    const OpticalFlow *optflow = AP::opticalflow();
+    const AP_OpticalFlow *optflow = AP::opticalflow();
 
     // exit immediately if no optical flow sensor or not healthy
     if (optflow == nullptr ||
@@ -3510,7 +3510,7 @@ void GCS_MAVLINK::handle_rc_channels_override(const mavlink_message_t &msg)
 #if AP_OPTICALFLOW_ENABLED
 void GCS_MAVLINK::handle_optical_flow(const mavlink_message_t &msg)
 {
-    OpticalFlow *optflow = AP::opticalflow();
+    AP_OpticalFlow *optflow = AP::opticalflow();
     if (optflow == nullptr) {
         return;
     }