boards/native: make compilation of qdec mock conditional

Previously this code was compiled unconditionally which might result in
a linking error since qdecs_value is not defined if the periph_qdec
module isn't enabled.

boards/native/drivers/native-qdec.c

@@ -20,6 +20,8 @@
 #include <board.h>
 #include <log.h>
 
+#ifdef MODULE_PERIPH_QDEC
+
 extern int32_t qdecs_value[QDEC_NUMOF];
 
 void native_motor_driver_qdec_simulation(
@@ -52,3 +54,5 @@ void native_motor_driver_qdec_simulation(
             motor_driver, motor_id);
     }
 }
+
+#endif /* MODULE_PERIPH_QDEC */

boards/native/include/board.h

@@ -141,7 +141,7 @@ void native_motor_driver_qdec_simulation( \
     int32_t pwm_duty_cycle);
 
 /* C++ standard do not support designated initializers */
-#ifndef __cplusplus
+#if !(defined __cplusplus) && (defined MODULE_PERIPH_QDEC)
 
 /**
  * @name Describe DC motors with PWM channel and GPIOs