env_dpdk/Makefile: use _uniq for pkg-config file

Including rte_net twice in DPDK_LIB_LIST results in
it also getting added twice to the spdk_env_dpdk.pc
pkg-config file.

Some (most?) pkg-config versions will detect the
duplicate entry and squash it. But some don't, which
results in multiple definition errors when linking
using pkg-config to get the library lists.

The native SPDK build uses a make function "_uniq"
to remove these duplicates. Use that same function here
to remove any duplicates for pkg-config file generation.

Fixes issue #3177.
Fixes f961b323 ("env/dpdk: add rte_net dependency to vhost")

Signed-off-by: Jim Harris <jim.harris@samsung.com>
Change-Id: If18c91aaa7a695903a595ebef0be1c07fa52316d
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/20561


Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Mellanox Build Bot
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>