cmake: define dpdk_LIBRARIES properly

cmake: define dpdk_LIBRARIES properly

dpdk_LIBRARIES should not be a list of dpdk library target, it should be
a list of paths to them. this also align with the definition of
`Finddpdk.cmake` in seastar project. so, if `Seastar_DPDK` is defined,
we should be able to offer the expected `dpdk_LIBRARIES` for seastar, as
our `Finddpdk.cmake` has higher priority than the one in seastar, and is
used when `find_package(dpdk...)` is called.

Signed-off-by: Kefu Chai <kchai@redhat.com>