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>

arch/powerpc/kernel/signal.h

@@ -49,6 +49,11 @@ extern int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
 
 #else /* CONFIG_PPC64 */
 
+extern long sys_rt_sigreturn(int r3, int r4, int r5, int r6, int r7, int r8,
+		     struct pt_regs *regs);
+extern long sys_sigreturn(int r3, int r4, int r5, int r6, int r7, int r8,
+		       struct pt_regs *regs);
+
 static inline int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
 				     struct task_struct *tsk)
 {