[MINOR] change type of fdtab[]->owner to void*

The owner of an fd was initially a task but this was sometimes
casted to a (struct listener *). We'll soon need more types,
so void* is more appropriate.
diff --git a/src/client.c b/src/client.c
index 5ff1abe..1f577d1 100644
--- a/src/client.c
+++ b/src/client.c
@@ -60,7 +60,7 @@
  * It returns 0.
  */
 int event_accept(int fd) {
-	struct listener *l = (struct listener *)fdtab[fd].owner;
+	struct listener *l = fdtab[fd].owner;
 	struct proxy *p = (struct proxy *)l->private; /* attached frontend */
 	struct session *s;
 	struct http_txn *txn;