AP_GyroFFT: Add missing const in member functions

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

libraries/AP_GyroFFT/AP_GyroFFT.cpp

@@ -841,7 +841,7 @@ void AP_GyroFFT::write_log_messages()
 // @Field: EnZ: power spectral density bin energy of the peak on roll
 
 // write a single log message
-void AP_GyroFFT::log_noise_peak(uint8_t id, FrequencyPeak peak, float notch)
+void AP_GyroFFT::log_noise_peak(uint8_t id, FrequencyPeak peak, float notch) const
 {
     AP::logger().Write("FTN2", "TimeUS,Id,PkX,PkY,PkZ,DnF,BwX,BwY,BwZ,EnX,EnY,EnZ", "s#zzzzzzz---", "F-----------", "QBffffffffff",
         AP_HAL::micros64(),

libraries/AP_GyroFFT/AP_GyroFFT.h

@@ -176,7 +176,7 @@ private:
         return (_thread_state._center_bandwidth_hz_filtered[peak][axis] = _center_bandwidth_filter[peak].apply(axis, value));
     }
     // write single log mesages
-    void log_noise_peak(uint8_t id, FrequencyPeak peak, float notch_freq);
+    void log_noise_peak(uint8_t id, FrequencyPeak peak, float notch_freq) const;
     // calculate the peak noise frequency
     void calculate_noise(bool calibrating, const EngineConfig& config);
     // calculate noise peaks based on energy and history