find_package(dpdk...) is used by seastar to find DPDK if Seastar_DPDK is
enabled. but `build_dpdk()` only exposes `dpdk::dpdk`, and it does not
set the variables like `dpdk_INCLUDE_DIRS` and `dpdk_LIBRARIES` when it
gets called. so we need to adapt Finddpdk to query them from `dpdk::dpdk`
target.
Signed-off-by: Kefu Chai <kchai@redhat.com>