powerpc/32: Mark both tmp variables as unused

Since the value of `tmp` is never intended to be read, declare both `tmp`
variables as unused. Fix warning (treated as error in W=1):

  arch/powerpc/kernel/signal_32.c: In function ‘sys_swapcontext’:
  arch/powerpc/kernel/signal_32.c:1048:16: error: variable ‘tmp’ set but not used
  arch/powerpc/kernel/signal_32.c: In function ‘sys_debug_setcontext’:
  arch/powerpc/kernel/signal_32.c:1234:16: error: variable ‘tmp’ set but not used

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>