arch/x86/kernel/ptrace.c: Quiet sparse noise

ptrace_set_debugreg() is only used in this file and should be
static.  This also quiets the following sparse warning:

  warning: symbol 'ptrace_set_debugreg' was not declared. Should it be static?

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: hartleys@visionengravers.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>