ixgbe: fix build of standard Rx with vectorized Tx

ixgbe: fix build of standard Rx with vectorized Tx

The vpmd RX routine won't be used if RTE_LIBRTE_IXGBE_RX_ALLOW_BULK_ALLOC
is not defined or its condition check fail.
The case RTE_LIBRTE_IXGBE_RX_ALLOC_BULK_ALLOC=n and RTE_IXGBE_INC_VECTOR=y
may exist when choose to use standard RX and optimized TX
(ixgbe_xmit_pkts_vec/ixgbe_xmit_pkts_simple).

Signed-off-by: Cunming Liang <cunming.liang@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Yong Liu <yong.liu@intel.com>
Tested-by: Zhaochen Zhan <zhaochen.zhan@intel.com>