linux-next: build warning after merge of the mm tree

Hi all,

After merging the mm tree, today's linux-next build (powerpc allyesconfig)
produced this warning:

arch/powerpc/kernel/swsusp_64.c:14:6: warning: no previous prototype for 'do_after_copyback' [-Wmissing-prototypes]
   14 | void do_after_copyback(void)
      |      ^~~~~~~~~~~~~~~~~

Exposed by commit

  c6345dfa6e3e ("Makefile.extrawarn: turn on missing-prototypes globally")

-- 
Cheers,
Stephen Rothwell