AP_HAL_Empty: add placeholder vector addition

libraries/AP_HAL_Empty/DSP.h

@@ -28,5 +28,6 @@ protected:
     virtual void vector_max_float(const float* vin, uint16_t len, float* maxValue, uint16_t* maxIndex) const override {}
     virtual void vector_scale_float(const float* vin, float scale, float* vout, uint16_t len) const override {}
     virtual float vector_mean_float(const float* vin, uint16_t len) const override { return 0.0f; };
+    virtual void vector_add_float(const float* vin1, const float* vin2, float* vout, uint16_t len) const override {};
 #endif // HAL_WITH_DSP
 };