kprobes/x86: Move skip_singlestep up

I get this warning:

  arch/x86/kernel/kprobes.c:544:23: warning: ‘skip_singlestep’ declared ‘static’ but never defined

on tip/auto-latest.

Put the skip_singlestep function declaration up, in
KPROBES_CAN_USE_FTRACE and drop the superfluous forward
declaration.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1348145034-16603-1-git-send-email-bp@amd64.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>