m68k: 68000: fix warning in timer code

When building with W=1:

      CC      arch/m68k/68000/timers.o
    arch/m68k/68000/timers.c:120:5: warning: no previous prototype for ‘m68328_hwclk’ [-Wmissing-prototypes]
     int m68328_hwclk(int set, struct rtc_time *t)
         ^~~~~~~~~~~~

Include m68328.h to get prototype for m68328_hwclk().

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>