arch/avr32: fix build failure caused by wrong prototype

arch/avr32: fix build failure caused by wrong prototype

This patch fixes a build failure introduced by 70ccd50ad ("avr32: use
generic ptrace_resume code") which had the static keyword as a leftover.

  arch/avr32/kernel/ptrace.c:32: error: static declaration of `user_enable_single_step' follows non-static declaration
  include/linux/ptrace.h:268: error: previous declaration of `user_enable_single_step' was here

References:
[1]http://kisskb.ellerman.id.au/kisskb/buildresult/2448162/

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>