POSIX: Avoid missing prototype warning on some POSIX platforms

src/drivers/device/vdev_posix.cpp

@@ -437,6 +437,11 @@ extern "C" {
 		return sim_delay;
 	}
 
+	bool px4_board_pwr(bool on)
+	{
+		return false;
+	}
+
 	const char *px4_get_device_names(unsigned int *handle)
 	{
 		return VDev::devList(handle);

src/platforms/px4_posix.h

@@ -116,7 +116,7 @@ __EXPORT void		px4_enable_sim_lockstep(void);
 __EXPORT void		px4_sim_start_delay(void);
 __EXPORT void		px4_sim_stop_delay(void);
 __EXPORT bool		px4_sim_delay_enabled(void);
-__EXPORT inline bool		px4_board_pwr(bool on) { return false; }
+__EXPORT bool		px4_board_pwr(bool on);
 
 __END_DECLS
 #else