[MINOR] session: add a pointer to the tracked counters for the source

We'll have to keep counters of various criteria specific to the session's
source. When we get one, keep a pointer to it in the session.
diff --git a/include/types/session.h b/include/types/session.h
index fd86f09..fdccc7b 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -184,6 +184,7 @@
 		int flags;
 	} store[8];				/* tracked stickiness values to store */
 	int store_count;
+	struct stksess *tracked_src_counters;	/* tracked counters for this source */
 
 	struct {
 		int logwait;			/* log fields waiting to be collected : LW_* */