m68k: fix conditional use of init_pointer_table

Compiling 3.8-rc1 fails for some m68k targets (the non-mmu ones) with:

  CC      arch/m68k/mm/init.o
arch/m68k/mm/init.c: In function ‘mem_init’:
arch/m68k/mm/init.c:191:2: error: implicit declaration of function ‘init_pointer_table’
arch/m68k/mm/init.c:191:36: error: ‘kernel_pg_dir’ undeclared (first use in this function)
arch/m68k/mm/init.c:191:36: note: each undeclared identifier is reported only once for each function it appears in
arch/m68k/mm/init.c:192:18: error: ‘PTRS_PER_PGD’ undeclared (first use in this function)
arch/m68k/mm/init.c:194:4: error: implicit declaration of function ‘__pgd_page’
arch/m68k/mm/init.c:198:6: error: ‘zero_pgtable’ undeclared (first use in this function)
make[2]: *** [arch/m68k/mm/init.o] Error 1
make[1]: *** [arch/m68k/mm] Error 2
make[1]: Leaving directory `/home/gerg/new-wave.git/linux-3.x'
make: *** [linux] Error 1

Change the conditions that define init_pointer_table so that it matches what
actually uses it.

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