m68k: fix inclusion of arch_gettimeoffset for non-MMU 68k classic CPU types

m68k: fix inclusion of arch_gettimeoffset for non-MMU 68k classic CPU types

When building for non-MMU based classic 68k CPU types (like the 68328 for
example) you get a compilation error:

  CC      arch/m68k/kernel/time.o
arch/m68k/kernel/time.c:91:5: error: redefinition of ‘arch_gettimeoffset’
include/linux/time.h:145:19: note: previous definition of ‘arch_gettimeoffset’ was here

The arch_gettimeoffset() code is included when building for these CPU types,
but it shouldn't be. Those machine types do not have
CONFIG_ARCH_USES_GETTIMEOFFSET set.

The fix is simply to conditionally include the arch_gettimeoffset() code on
that same config setting that specifies its use or not.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>