sh: fix ptrace_64.c:user_disable_single_step()

This patch fixes the following compile error caused by
commit c459dbf2
(sh: ptrace single stepping cleanups.):

<--  snip  -->

...
  CC      arch/sh/kernel/ptrace_64.o
arch/sh/kernel/ptrace_64.c: In function 'user_disable_single_step':
arch/sh/kernel/ptrace_64.c:134: error: 'regs' undeclared (first use in this function)
arch/sh/kernel/ptrace_64.c:134: error: (Each undeclared identifier is reported only once
arch/sh/kernel/ptrace_64.c:134: error: for each function it appears in.)
...
make[2]: *** [arch/sh/kernel/ptrace_64.o] Error 1

<--  snip  -->

Reported-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>