i40e: fix build without Rx bulk alloc

i40e: fix build without Rx bulk alloc

Issue:
When CONFIG_RTE_LIBTRE_I40E_RX_ALLOW_BULK_ALLOC=n in config file, there
will be a build error:
	'i40e_recv_pkts_bulk_alloc' undeclared

Now DPDK i40e PMD uses the preprocessor to choose whether or not to define
the bulk recv functions, but for selection of the RX function, PMD only
depends on a C variable. This causes the inconsistency and leads to the
build error due to the bulk recv function not being defined.

Fixes: 8e109464c022 ("i40e: allow vector Rx and Tx usage")

Signed-off-by: Zhe Tao <zhe.tao@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>