powerpc: pmd_move_must_withdraw() is only needed for CONFIG_TRANSPARENT_HUGEPAGE

This is required for the later patch "Makefile.extrawarn: turn on
missing-prototypes globally".

Link: https://lkml.kernel.org/r/20231127132809.45c2b398@canb.auug.org.au


Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>