AP_L1_Control: Add missing const in member functions

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

libraries/AP_L1_Control/AP_L1_Control.cpp

@@ -53,7 +53,7 @@ const AP_Param::GroupInfo AP_L1_Control::var_info[] = {
 /*
   Wrap AHRS yaw if in reverse - radians
  */
-float AP_L1_Control::get_yaw()
+float AP_L1_Control::get_yaw() const
 {
     if (_reverse) {
         return wrap_PI(M_PI + _ahrs.yaw);

libraries/AP_L1_Control/AP_L1_Control.h

@@ -127,6 +127,6 @@ private:
     AP_Float _loiter_bank_limit;
 
     bool _reverse = false;
-    float get_yaw();
+    float get_yaw() const;
     int32_t get_yaw_sensor() const;
 };