clocksource: Implement clocksource_select_fallback() for CONFIG_ARCH_USES_GETTIMEOFFSET=y

commit 7eaeb343 (clocksource: Provide unbind interface in sysfs)
implemented clocksource_select_fallback() which is not defined for
CONFIG_ARCH_USES_GETTIMEOFFSET=y. Add an empty inline function for
that.

Reported-by: Ingo Molnar <mingo@kernel.org>
Reported-by:  <fengguang.wu@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>