powerpc: pmd_move_must_withdraw() is only needed for CONFIG_TRANSPARENT_HUGEPAGE

The linux-next build of powerpc64 allnoconfig fails with:

  arch/powerpc/mm/book3s64/pgtable.c:557:5: error: no previous prototype for 'pmd_move_must_withdraw'
    557 | int pmd_move_must_withdraw(struct spinlock *new_pmd_ptl,
        |     ^~~~~~~~~~~~~~~~~~~~~~

Caused by commit:

  c6345dfa6e3e ("Makefile.extrawarn: turn on missing-prototypes globally")

Fix it by moving the function definition under
CONFIG_TRANSPARENT_HUGEPAGE like the prototype. The function is only
called when CONFIG_TRANSPARENT_HUGEPAGE=y.
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
[mpe: Flesh out change log from linux-next patch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231127132809.45c2b398@canb.auug.org.au

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?