arm_arch_timer: include clocksource.h directly

This driver makes use of the clocksource code. Previously it had only
included the proper header indirectly, but that chain was inadvertently
broken by 74d23cc7 "time: move the timecounter/cyclecounter code into its
own file."

This patch fixes the issue by including clocksource.h directly.
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/clocksource/arm_arch_timer.c

@@ -15,6 +15,7 @@
 #include <linux/cpu.h>
 #include <linux/cpu_pm.h>
 #include <linux/clockchips.h>
+#include <linux/clocksource.h>
 #include <linux/interrupt.h>
 #include <linux/of_irq.h>
 #include <linux/of_address.h>