ARM: vexpress/hotplug: fix missing core.h include

Fix the missing declaration of vexpress_cpu_die() by
including core.h where it is defined. Fixes:

arch/arm/mach-vexpress/hotplug.c:88:6: warning: symbol 'vexpress_cpu_die' was not declared. Should it be static?

Cc: Liviu Dudau <[e-mail]>
Cc: Lorenzo Pieralisi <[e-mail]>
Signed-off-by: Ben Dooks <[e-mail]>
Signed-off-by: Sudeep Holla <[e-mail]>

arch/arm/mach-vexpress/hotplug.c

···
 #include <asm/smp_plat.h>
 #include <asm/cp15.h>
 
+#include "core.h"
+
 static inline void cpu_enter_lowpower(void)
 {
 	unsigned int v;