AP_WheelEncoder: Add missing const in member functions

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

libraries/AP_WheelEncoder/AP_WheelEncoder.cpp

@@ -202,7 +202,7 @@ void AP_WheelEncoder::update(void)
 }
 
 // log wheel encoder information
-void AP_WheelEncoder::Log_Write()
+void AP_WheelEncoder::Log_Write() const
 {
     // return immediately if no wheel encoders are enabled
     if (!enabled(0) && !enabled(1)) {

libraries/AP_WheelEncoder/AP_WheelEncoder.h

@@ -70,7 +70,7 @@ public:
     void update(void);
 
     // log data to logger
-    void Log_Write();
+    void Log_Write() const;
 
     // return the number of wheel encoder sensor instances
     uint8_t num_sensors(void) const { return num_instances; }