[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: Kumar Gala <[e-mail]>
Signed-off-by: Josh Boyer <[e-mail]>
Signed-off-by: Paul Mackerras <[e-mail]>

include/asm-ppc/system.h

···
 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);