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>

arch/powerpc/include/asm/asm-prototypes.h

@@ -89,6 +89,10 @@ int sys_swapcontext(struct ucontext __user *old_ctx,
 long sys_swapcontext(struct ucontext __user *old_ctx,
 		    struct ucontext __user *new_ctx,
 		    int ctx_size, int r6, int r7, int r8, struct pt_regs *regs);
+int sys_debug_setcontext(struct ucontext __user *ctx,
+			 int ndbg, struct sig_dbg_op __user *dbg,
+			 int r6, int r7, int r8,
+			 struct pt_regs *regs);
 #endif
 long sys_switch_endian(void);
 notrace unsigned int __check_irq_replay(void);