selftests: net: build net/lib dependency in all target

We have the logic to include net/lib automatically for net related
selftests. However, currently, this logic is only in install target
which means only `make install` will have net/lib included. This commit
adds the logic to all target so that all `make`, `make run_tests` and
`make install` will have net/lib included in net related selftests.

Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Link: https://patch.msgid.link/20250601142914.13379-1-minhquangbui99@gmail.com


Fixes: b86761ff ("selftests: net: add scaffolding for Netlink tests in Python")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>