exclude the AOA code if USE_AOA is not defined

sw/airborne/firmwares/fixedwing/stabilization/stabilization_attitude.h

@@ -56,10 +56,12 @@ extern float  h_ctl_roll_pgain;
 extern pprz_t h_ctl_aileron_setpoint;
 extern float  h_ctl_roll_slew;
 
+#ifdef USE_AOA
 /* Pitch mode */
 #define H_CTL_PITCH_MODE_THETA	0
 #define H_CTL_PITCH_MODE_AOA	1
 extern uint8_t h_ctl_pitch_mode;
+#endif
 
 /* inner pitch loop parameters */
 extern float  h_ctl_pitch_setpoint;