[bsp][efm32] Fix RTC driver compile error

bsp/efm32/drv_rtc.c

@@ -24,6 +24,8 @@
  ******************************************************************************/
 
 /* Includes ------------------------------------------------------------------*/
+
+#include <rtdevice.h>
 #include "board.h"
 #include "hdl_interrupt.h"
 #include "drv_rtc.h"