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>