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/lib/FlightTasks/tasks/Utility/ManualSmoothingXY.cpp

@@ -38,6 +38,7 @@
 #include "ManualSmoothingXY.hpp"
 #include <mathlib/mathlib.h>
 #include <float.h>
+#include <px4_defines.h>
 
 using namespace matrix;