sched/walt: include missing header for arm_timer_read_counter()

Include clocksource/arm_arch_timer.h to fix implicit function
declaration of ‘arch_timer_read_counter’ build error for ARCH=arm.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
[jstultz: Cherry-picked from common/android-3.18]
Signed-off-by: John Stultz <john.stultz@linaro.org>

kernel/sched/walt.c

@@ -22,6 +22,7 @@
 #include <linux/syscore_ops.h>
 #include <linux/cpufreq.h>
 #include <trace/events/sched.h>
+#include <clocksource/arm_arch_timer.h>
 #include "sched.h"
 #include "walt.h"