powerpc: Add missing prototype for sys_debug_setcontext()

In commit 81e7009e ("powerpc: merge ppc signal.c and ppc64
signal32.c") the function sys_debug_setcontext was added without a
prototype.

Fix compilation warning (treated as error in W=1):
  arch/powerpc/kernel/signal_32.c:1227:5: error: no previous prototype for ‘sys_debug_setcontext’

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