[POWERPC] Fix CONFIG_SMP=n build error on ppc64

The patch "KVM: fix !SMP build error" change the way smp_call_function()
actually uses the passed in function names on non-SMP builds.  So
previously it was never caught that the function passed in was never
actually defined.

This causes a build error on ppc64_defconfig + CONFIG_SMP=n:

arch/powerpc/mm/tlb_64.c: In function 'pgtable_free_now':
arch/powerpc/mm/tlb_64.c:71: error: 'pte_free_smp_sync' undeclared (first use in this function)
arch/powerpc/mm/tlb_64.c:71: error: (Each undeclared identifier is reported only once
arch/powerpc/mm/tlb_64.c:71: error: for each function it appears in.)

So we need to define it even if CONFIG_SMP is off. Either that or ifdef
out the smp_call_function() call, but that's ugly.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>