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>
diff --git a/src/main.c b/src/main.c
index 1591018..329db14 100644
--- a/src/main.c
+++ b/src/main.c
@@ -393,7 +393,9 @@
 		[CLOCK_MONOTONIC_RAW] =		"CLOCK_MONOTONIC_RAW",
 		[CLOCK_REALTIME_COARSE] =	"CLOCK_REALTIME_COARSE",
 		[CLOCK_MONOTONIC_COARSE] =	"CLOCK_MONOTONIC_COARSE",
+#ifdef CLOCK_BOOTTIME
 		[CLOCK_BOOTTIME] =		"CLOCK_BOOTTIME",
+#endif
 	};
 
 	if (clk_id < 0 || (unsigned)clk_id >= ARRAY_LENGTH(names))