rtc: make definitions in include/uapi/linux/rtc.h actually useful for user space

rtc: make definitions in include/uapi/linux/rtc.h actually useful for user space

BIT() macro is not defined in UAPI headers; there is, however, similarly
defined _BITUL() macro present in include/uapi/linux/const.h; use it
instead and include <linux/const.h> and <linux/ioctl.h> in order to make
the definitions provided in the header useful.

Fixes: 3431ca4837bf ("rtc: define RTC_VL_READ values")
Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Link: https://lore.kernel.org/r/20200324041209.GA30727@asgard.redhat.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>