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/battery/battery.cpp

@@ -42,6 +42,7 @@
 #include "battery.h"
 #include <mathlib/mathlib.h>
 #include <cstring>
+#include <px4_defines.h>
 
 Battery::Battery() :
 	ModuleParams(nullptr),