Keep RTC API prototypes even when not enabled

Keep the prototypes in rtc_api.h even when DEVICE_RTC is not defined.
This allows devices that aren't fully compliant with the RTC API to
still use the header and prototypes.

hal/rtc_api.h

@@ -21,8 +21,6 @@
 
 #include "device.h"
 
-#if DEVICE_RTC
-
 #include <time.h>
 
 #ifdef __cplusplus
@@ -72,6 +70,4 @@ void rtc_write(time_t t);
 
 #endif
 
-#endif
-
 /** @}*/