px4.h: remove this include, causes compile problems on GCC 6.1.0

It causes problems because it includes px4_nodehandle.h which in turn
includes <functional>, and this is not available on NuttX

src/platforms/nuttx/px4_nuttx_impl.cpp

@@ -37,7 +37,8 @@
  * PX4 Middleware Wrapper NuttX Implementation
  */
 
-#include <px4.h>
+#include <px4_defines.h>
+#include <px4_middleware.h>
 #include <drivers/drv_hrt.h>