use px4_defines header to distinguish platform

src/modules/uORB/topics/airspeed.h

@@ -40,9 +40,7 @@
 #ifndef TOPIC_AIRSPEED_H_
 #define TOPIC_AIRSPEED_H_
 
-#ifdef CONFIG_ARCH_ARM
-#include "../uORB.h"
-#endif
+#include <platforms/px4_defines.h>
 #include <stdint.h>
 
 /**
@@ -65,8 +63,6 @@ struct airspeed_s {
  */
 
 /* register this as object request broker structure */
-#ifdef CONFIG_ARCH_ARM
 ORB_DECLARE(airspeed);
-#endif
 
 #endif