rtc.h: add struct udevice declaration

Without this change, including rtc.h solely will cause a build error.
Signed-off-by: NAKASHI Takahiro <takahiro.akashi@linaro.org>

include/rtc.h

@@ -18,6 +18,8 @@
 
 #ifdef CONFIG_DM_RTC
 
+struct udevice;
+
 struct rtc_ops {
 	/**
 	 * get() - get the current time