ToneAlarmInterface: move to drivers/drv_tone_alarm

It belongs there since it's not a library providing the implementation, but
just declaring the interface.

src/drivers/drv_tone_alarm.h

@@ -91,3 +91,23 @@ enum {
 	TONE_HOME_SET,
 	TONE_NUMBER_OF_TUNES
 };
+
+namespace ToneAlarmInterface
+{
+
+/**
+ * @brief Activates/configures the hardware registers.
+ */
+void init();
+
+/**
+ * @brief Starts playing the note.
+ */
+void start_note(unsigned frequency);
+
+/**
+ * @brief Stops playing the current note and makes the player 'safe'.
+ */
+void stop_note();
+
+} // ToneAlarmInterface

src/drivers/tone_alarm/ToneAlarm.h

@@ -43,7 +43,6 @@
 #include <circuit_breaker/circuit_breaker.h>
 #include <drivers/device/device.h>
 #include <drivers/drv_tone_alarm.h>
-#include <lib/drivers/tone_alarm/ToneAlarmInterface.h>
 #include <lib/tunes/tunes.h>
 #include <px4_defines.h>
 #include <px4_work_queue/ScheduledWorkItem.hpp>