sensors: initialize adc only if BOARD_NUMBER_BRICKS > 0

Fixes the SITL startup error:
ERROR [sensors] no ADC found: /dev/adc0 (9)

Introduced in 8404889

src/modules/sensors/sensors.cpp

@@ -582,7 +582,7 @@ void
 Sensors::run()
 {
 	if (!_hil_enabled) {
-#if !defined(__PX4_QURT) && !defined(__PX4_POSIX_BEBOP)
+#if !defined(__PX4_QURT) && BOARD_NUMBER_BRICKS > 0
 		adc_init();
 #endif
 	}