MINOR: quic: Add a pool for TX ring buffer internal buffer

We want to allocate the internal buffer of TX ring buffer from a pool.
This patch add "quic_tx_ring_pool" to do so.
diff --git a/src/xprt_quic.c b/src/xprt_quic.c
index ddffd10..cfd8a68 100644
--- a/src/xprt_quic.c
+++ b/src/xprt_quic.c
@@ -130,6 +130,7 @@
 
 static BIO_METHOD *ha_quic_meth;
 
+DECLARE_POOL(pool_head_quic_tx_ring, "quic_tx_ring_pool", QUIC_TX_RING_BUFSZ);
 DECLARE_STATIC_POOL(pool_head_quic_conn_ctx,
                     "quic_conn_ctx_pool", sizeof(struct ssl_sock_ctx));
 DECLARE_STATIC_POOL(pool_head_quic_conn, "quic_conn", sizeof(struct quic_conn));