ARM: vexpress/spc: fix missing include of spc.h

Fix missing function prototypes found in spc.h by including
the file to remove the following warnings:

arch/arm/mach-vexpress/spc.c:131:6: warning: symbol 've_spc_global_wakeup_irq' was not declared. Should it be static?
arch/arm/mach-vexpress/spc.c:156:6: warning: symbol 've_spc_cpu_wakeup_irq' was not declared. Should it be static?
arch/arm/mach-vexpress/spc.c:185:6: warning: symbol 've_spc_set_resume_addr' was not declared. Should it be static?
arch/arm/mach-vexpress/spc.c:210:6: warning: symbol 've_spc_powerdown' was not declared. Should it be static?
arch/arm/mach-vexpress/spc.c:240:5: warning: symbol 've_spc_cpu_in_wfi' was not declared. Should it be static?
arch/arm/mach-vexpress/spc.c:450:12: warning: symbol 've_spc_init' 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>