Fix ARCH=ppc builds

The recent signal rework broke ARCH=ppc builds with the following error:

CC      arch/powerpc/kernel/signal.o
arch/powerpc/kernel/signal.c: In function ‘do_signal’:
arch/powerpc/kernel/signal.c:142: error: implicit declaration of function ‘set_dabr’
make[1]: *** [arch/powerpc/kernel/signal.o] Error 1

The patch below fixes it by including a function prototype in
asm-ppc/system.h

Signed-off-by: Josh Boyer <redacted>

diff --git a/include/asm-ppc/system.h b/include/asm-ppc/system.h
index f1311a8..cc45780 100644
--- a/include/asm-ppc/system.h
+++ b/include/asm-ppc/system.h

@@ -54,6 +54,7 @@ extern void show_regs(struct pt_regs * regs);
 extern void flush_instruction_cache(void);
 extern void hard_reset_now(void);
 extern void poweroff_now(void);
+extern int set_dabr(unsigned long dabr);
 #ifdef CONFIG_6xx
 extern long _get_L2CR(void);
 extern long _get_L3CR(void);