powerpc: Add missing sparsemem.h include

arch/powerpc/platforms/pseries/hotplug-memory.c uses
remove_section_mapping() but doesn't include sparsemem.h which defines
it.  This can cause compilation fails for some configs.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>