arm: arch_timer: add missing inline in stub function

Fixes:

In file included from arch/arm/include/asm/arch_timer.h:10:0,
                 from arch/arm/mach-shmobile/timer.c:23:
include/clocksource/arm_arch_timer.h:56:28: warning: 'arch_timer_get_timecounter' defined but not used [-Wunused-function]

Signed-off-by: Olof Johansson <olof@lixom.net>