ARM: 8913/1: arch_timer: include <asm/arch_timer.h>

The arch_timer_arch_init is defined in <asm/arch_timer.h> so include
that to fix the following sparse error:

arch/arm/kernel/arch_timer.c:31:12: warning: symbol 'arch_timer_arch_init' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>