m68k/bvme6000: Include generic <linux/rtc.h>

The BVME6000 RTC driver doesn't use any MC146818-specific definitions,
hence include the generic <linux/rtc.h> instead of
<linux/mc146818rtc.h>.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>