config: enable packet prefetching with Meson

config: enable packet prefetching with Meson

With Make build system, RTE_PMD_PACKET_PREFETCH was enabled
by default. It got lost when transitioning to Meson build
system.

In order to avoid performance changes, this patch enables
packet prefetching in rte_config.h.

Fixes: 9314afb68a53 ("drivers: add infrastructure for meson build")
Cc: stable@dpdk.org
Reported-by: Marvin Liu <yong.liu@intel.com>
Suggested-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>