AP_Airspeed: add AP::airspeed() to get airspeed singleton

libraries/AP_Airspeed/AP_Airspeed.cpp

@@ -501,3 +501,12 @@ bool AP_Airspeed::all_healthy(void) const
 
 // singleton instance
 AP_Airspeed *AP_Airspeed::_singleton;
+
+namespace AP {
+
+AP_Airspeed *airspeed()
+{
+    return AP_Airspeed::get_singleton();
+}
+
+};

libraries/AP_Airspeed/AP_Airspeed.h

@@ -248,3 +248,7 @@ private:
     AP_Airspeed_Backend *sensor[AIRSPEED_MAX_SENSORS];
 
 };
+
+namespace AP {
+    AP_Airspeed *airspeed();
+};