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>

include/clocksource/arm_arch_timer.h

@@ -53,7 +53,7 @@ static inline u64 arch_timer_read_counter(void)
 	return 0;
 }
 
-static struct timecounter *arch_timer_get_timecounter(void)
+static inline struct timecounter *arch_timer_get_timecounter(void)
 {
 	return NULL;
 }