clocksource: Add missing include of of.h.

This header uses OF_DELCARE_1 which is defined in linux/of.h.

This fixes getting unhelpful compiler error messages about missing ')'
before a string constant.

Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

include/linux/clocksource.h

@@ -15,6 +15,7 @@
 #include <linux/cache.h>
 #include <linux/timer.h>
 #include <linux/init.h>
+#include <linux/of.h>
 #include <asm/div64.h>
 #include <asm/io.h>