[POWERPC] 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

This fixes it by including a function prototype in asm-ppc/system.h.
Acked-by: NKumar Gala <galak@kernel.crashing.org>
Signed-off-by: NJosh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

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);