[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 <[e-mail]>
Signed-off-by: Paul Mackerras <[e-mail]>

arch/ppc/kernel/ppc_ksyms.c

···
 long long __ashldi3(long long, int);
 long long __lshrdi3(long long, int);
 
+EXPORT_SYMBOL(empty_zero_page);
 EXPORT_SYMBOL(clear_pages);
 EXPORT_SYMBOL(clear_user_page);
+EXPORT_SYMBOL(copy_page);
 EXPORT_SYMBOL(transfer_to_handler);
 EXPORT_SYMBOL(do_IRQ);
 EXPORT_SYMBOL(machine_check_exception);