powerpc/suspend: Add prototype for do_after_copyback()

With HIBERNATION=y the build breaks with:

  arch/powerpc/kernel/swsusp_64.c:14:6: error: no previous prototype for ‘do_after_copyback’ [-Werror=missing-prototypes]
  14 | void do_after_copyback(void)
     |      ^~~~~~~~~~~~~~~~~

do_after_copyback() is only called from asm, so there is no prototype,
nor any header where it makes sense to place one. Just add a prototype
in the C file to fix the build error.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231129131919.2528517-1-mpe@ellerman.id.au