x86: Mark ptrace_get_debugreg() as static

This sparse warning:

  arch/x86/kernel/ptrace.c:560:15: warning: symbol 'ptrace_get_debugreg' was not declared. Should it be static?

triggers because ptrace_get_debugreg() is global but is only
used in a single .c file. change ptrace_get_debugreg() to
static to fix that - this also addresses the sparse warning.

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <1246458150.6940.19.camel@hpdv5.satnam>
Signed-off-by: Ingo Molnar <mingo@elte.hu>