powerpc: Add missing prototypes for sys_sigreturn() & sys_rt_sigreturn()

Two functions did not have a prototype defined in signal.h header. Fix
the following two warnings (treated as errors in W=1):

  arch/powerpc/kernel/signal_32.c:1135:6: error: no previous prototype for ‘sys_rt_sigreturn’
  arch/powerpc/kernel/signal_32.c:1422:6: error: no previous prototype for ‘sys_sigreturn’

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