rtc: Move RTC function prototypes to their own header

softmmu/rtc.c defines two public functions: qemu_get_timedate() and
qemu_timedate_diff().  Currently we keep the prototypes for these in
qemu-common.h, but most files don't need them.  Move them to their
own header, a new include/sysemu/rtc.h.

Since the C files using these two functions did not need to include
qemu-common.h for any other reason, we can remove those include lines
when we add the include of the new rtc.h.

The license for the .h file follows that of the softmmu/rtc.c
where both the functions are defined.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>