BUG/MINOR: stats: fix missing comma in stats on agent drain

The csv stats format breaks when agent changes server state to drain.
Tools like hatop, metric or check agents will fail due to this. This
should be backported to 1.6.
diff --git a/src/dumpstats.c b/src/dumpstats.c
index f43a5cc..b868ef5 100644
--- a/src/dumpstats.c
+++ b/src/dumpstats.c
@@ -3584,7 +3584,7 @@
 			[SRV_STATS_STATE_NOLB]			= "NOLB,",
 			[SRV_STATS_STATE_DRAIN_GOING_DOWN]	= "DRAIN %d/%d,",
 			[SRV_STATS_STATE_DRAIN]			= "DRAIN,",
-			[SRV_STATS_STATE_DRAIN_AGENT]		= "DRAIN (agent)",
+			[SRV_STATS_STATE_DRAIN_AGENT]		= "DRAIN (agent),",
 			[SRV_STATS_STATE_NO_CHECK]		= "no check,"
 		};