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 <liviu.dudau@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>