compositor/clients: Protect CLOCK_BOOTTIME with ifdefs

CLOCK_BOOTTIME is a relatively new* feature that may not actually be
present everywhere (I'm looking at you wheezy).  Since our use of it
is actually only cosmetic, I've just ifdef'd if.

* No it isn't.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>