packet: Use flexible array member in struct pool

Currently we have a dummy pkt[1] array, which we alias with an array of
a different size via various macros.  However, we already require C11 which
includes flexible array members, so we can do better.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

packet.h

@@ -21,7 +21,7 @@ struct pool {
 	size_t buf_size;
 	size_t size;
 	size_t count;
-	struct iovec pkt[1];
+	struct iovec pkt[];
 };
 
 int vu_packet_check_range(void *buf, size_t offset, size_t len,