AP_Math: move HALF_SQRT_2 def to rotation.h

This allows it to be used by the Quaternion class

libraries/AP_Math/rotations.h

@@ -80,6 +80,9 @@ enum Rotation : uint8_t {
 // maximum rotation that will be used for auto-detection
 #define ROTATION_MAX_AUTO_ROTATION ROTATION_ROLL_90_PITCH_315
 
+// definitions used by quaterion and vector3f
+#define HALF_SQRT_2 0.70710678118654757f
+
 /*
 Here are the same values in a form suitable for a @Values attribute in
 auto documentation:

libraries/AP_Math/vector3.cpp

@@ -20,8 +20,6 @@
 
 #include "AP_Math.h"
 
-#define HALF_SQRT_2 0.70710678118654757f
-
 // rotate a vector by a standard rotation, attempting
 // to use the minimum number of floating point operations
 template <typename T>