[MINOR] stick_table: add support for "conn_cum" data type.

This one can be parsed on the "stick-table" after with the "store"
keyword. It will hold the number of connections matching the entry,
for use with ACLs or anything else.
diff --git a/src/stick_table.c b/src/stick_table.c
index 158056e..4cd6c75 100644
--- a/src/stick_table.c
+++ b/src/stick_table.c
@@ -477,6 +477,7 @@
 
 /* Extra data types processing */
 struct stktable_data_type stktable_data_types[STKTABLE_DATA_TYPES] = {
+	[STKTABLE_DT_CONN_CUM] = { .name = "conn_cum", .data_length = stktable_data_size(conn_cum) },
 };
 
 /*