ixgbe: fix build with gcc 4.4

The refcnt field is contained within an anonymous union within the mbuf
data structure, and gcc 4.4 gives an error about an unknown field unless
the initialiser for the field is contained within extra braces.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>

lib/librte_pmd_ixgbe/ixgbe_rxtx_vec.c

@@ -723,7 +723,7 @@ ixgbe_rxq_vec_setup(struct igb_rx_queue *rxq)
 		.nb_segs = 1,
 		.data_off = RTE_PKTMBUF_HEADROOM,
 #ifdef RTE_MBUF_REFCNT
-		.refcnt = 1,
+		{ .refcnt = 1, }
 #endif
 	};