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.auSigned-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>

arch/powerpc/mm/book3s64/pgtable.c

@@ -542,6 +542,7 @@ void ptep_modify_prot_commit(struct vm_area_struct *vma, unsigned long addr,
 	set_pte_at(vma->vm_mm, addr, ptep, pte);
 }
 
+#ifdef CONFIG_TRANSPARENT_HUGEPAGE
 /*
  * For hash translation mode, we use the deposited table to store hash slot
  * information and they are stored at PTRS_PER_PMD offset from related pmd
@@ -563,6 +564,7 @@ int pmd_move_must_withdraw(struct spinlock *new_pmd_ptl,
 
 	return true;
 }
+#endif
 
 /*
  * Does the CPU support tlbie?