[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/proto_tcp.c b/src/proto_tcp.c
index 0cf1e0e..32c4994 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -263,7 +263,7 @@
 	fdtab[fd].cb[DIR_RD].f = listener->accept;
 	fdtab[fd].cb[DIR_WR].f = NULL; /* never called */
 	fdtab[fd].cb[DIR_RD].b = fdtab[fd].cb[DIR_WR].b = NULL;
-	fdtab[fd].owner = (struct task *)listener; /* reference the listener instead of a task */
+	fdtab[fd].owner = listener; /* reference the listener instead of a task */
 	fdtab[fd].state = FD_STLISTEN;
 	fdtab[fd].peeraddr = NULL;
 	fdtab[fd].peerlen = 0;