MINOR: connections: Make the "list" element a struct mt_list instead of list.

Make the "list" element a struct mt_list, and explicitely use
list_from_mt_list to get a struct list * where it is used as such, so that
mt_list_for_each_entry will be usable with it.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 47b9068..0d42324 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -3906,9 +3906,9 @@
 					return;
 
 				}
-			} else if (LIST_ISEMPTY(&h2c->conn->list) &&
+			} else if (MT_LIST_ISEMPTY(&h2c->conn->list) &&
 			           h2_avail_streams(h2c->conn) > 0 && objt_server(h2c->conn->target)) {
-				LIST_ADD(&__objt_server(h2c->conn->target)->available_conns[tid], &h2c->conn->list);
+				LIST_ADD(&__objt_server(h2c->conn->target)->available_conns[tid], mt_list_to_list(&h2c->conn->list));
 			}
 		}
 	}