powerpc/mem: Add back missing header to fix 'no previous prototype' error

Commit b26e8f27253a ("powerpc/mem: Move cache flushing functions into
mm/cacheflush.c") removed asm/sparsemem.h which is required when
CONFIG_MEMORY_HOTPLUG is selected to get the declaration of
create_section_mapping().

Add it back.

Fixes: b26e8f27253a ("powerpc/mem: Move cache flushing functions into mm/cacheflush.c")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: Christophe Leroy <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/3e5b63bb[e-mail]

arch/powerpc/mm/mem.c

···
 #include <asm/machdep.h>
 #include <asm/rtas.h>
 #include <asm/kasan.h>
+#include <asm/sparsemem.h>
 #include <asm/svm.h>
 
 #include <mm/mmu_decl.h>