ARM: 9219/1: fix undeclared soft_restart

The soft_restart() is declared in <asm/system_misc.h> so
include that to fix the following sparse warning:

arch/arm/kernel/reboot.c:78:6: warning: symbol 'soft_restart' was not declared. Should it be static?
Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

arch/arm/kernel/reboot.c

@@ -10,6 +10,7 @@
 #include <asm/cacheflush.h>
 #include <asm/idmap.h>
 #include <asm/virt.h>
+#include <asm/system_misc.h>
 
 #include "reboot.h"