mixer: minor refactoring to reduce header include dependencies

- avoid including <px4_defines.h> from the math headers
- avoid driver include <drivers/drv_mixer.h> from the mixer

src/modules/mc_pos_control/PositionControl.cpp

@@ -39,6 +39,7 @@
 #include <float.h>
 #include <mathlib/mathlib.h>
 #include "Utility/ControlMath.hpp"
+#include <px4_defines.h>
 
 using namespace matrix;