clocksource: add empty version of clocksource_of_init

Add an empty clocksource_of_init when !CLKSRC_OF. This is needed for builds
where no timer has selected CLKSRC_OF.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

include/linux/clocksource.h

@@ -340,6 +340,7 @@ extern void clocksource_of_init(void);
 		__used __section(__clksrc_of_table)			\
 		 = { .compatible = compat, .data = fn };
 #else
+static inline void clocksource_of_init(void) {}
 #define CLOCKSOURCE_OF_DECLARE(name, compat, fn)
 #endif