[POWERPC] Export empty_zero_page and copy_page in arch/ppc

Currently ext4 and fuse fail to link if modular:
ERROR: "copy_page" [fs/fuse/fuse.ko] undefined!
ERROR: "empty_zero_page" [fs/ext4/ext4dev.ko] undefined!
make[3]: *** [__modpost] Error 1
make[2]: *** [modules] Error 2
make[1]: *** [sub-make] Error 2

While arch ppc exists it may as well compile, so this exports those
symbols (which are already exported in arch/powerpc).

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>