Fixes: https://github.com/libopencm3/libopencm3/issues/1446
Fixes: 18f4d7c1b7 stm32f3: rtc: include correct shared header
No testing on real hardware, but the docs imply that this should have
always had the "basic" rtc-v2 peripheral.
Signed-off-by: Karl Palsson <karlp@tweak.net.au>