powerpc/mm/radix: Make radix__change_memory_range() static

The lkp bot pointed out that with W=1 we get:

  arch/powerpc/mm/book3s64/radix_pgtable.c:183:6: error: no previous
  prototype for 'radix__change_memory_range'

Which is really saying that it could be static, make it so.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/book3s64/radix_pgtable.c

@@ -180,8 +180,8 @@ int radix__map_kernel_page(unsigned long ea, unsigned long pa,
 }
 
 #ifdef CONFIG_STRICT_KERNEL_RWX
-void radix__change_memory_range(unsigned long start, unsigned long end,
-				unsigned long clear)
+static void radix__change_memory_range(unsigned long start, unsigned long end,
+				       unsigned long clear)
 {
 	unsigned long idx;
 	pgd_t *pgdp;