AP_Math: fixed expo_curve()

doesn't make sense as constexpr

libraries/AP_Math/AP_Math.cpp

@@ -118,7 +118,7 @@ float linear_interpolate(float low_output, float high_output,
  * alpha range: [0,1] min to max expo
  * input range: [-1,1]
  */
-constexpr float expo_curve(float alpha, float x)
+float expo_curve(float alpha, float x)
 {
     return (1.0f - alpha) * x + alpha * x * x * x;
 }

libraries/AP_Math/AP_Math.h

@@ -271,7 +271,7 @@ float linear_interpolate(float low_output, float high_output,
  * alpha range: [0,1] min to max expo
  * input range: [-1,1]
  */
-constexpr float expo_curve(float alpha, float input);
+float expo_curve(float alpha, float input);
 
 /* throttle curve generator
  * thr_mid: output at mid stick