vmalloc: pcpu_get/free_vm_areas() aren't needed on UP

These functions are used only by percpu memory allocator on SMP.
Don't build them on UP.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Nick Piggin <npiggin@kernel.dk>
Reviewed-by: Chrsitoph Lameter <cl@linux.com>