powerpc/mm: remove warning about ‘type’ being set

‘type’ is only used when CONFIG_DEBUG_HIGHMEM is set. So add a possibly
unused tag to variable. Remove warning treated as error with W=1:

  arch/powerpc/mm/highmem.c:59:6: error: variable ‘type’ set but not used [-Werror=unused-but-set-variable]
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/highmem.c

@@ -56,7 +56,7 @@ EXPORT_SYMBOL(kmap_atomic_prot);
 void __kunmap_atomic(void *kvaddr)
 {
 	unsigned long vaddr = (unsigned long) kvaddr & PAGE_MASK;
-	int type;
+	int type __maybe_unused;
 
 	if (vaddr < __fix_to_virt(FIX_KMAP_END)) {
 		pagefault_enable();