Fix cfg flag for tcp tests

[SabatierBoris]

Issue

tcp test for serde_transport isn't run due to wrong cfg flag

Fix

Use same cfg flag for test as for the tcp module : https://github.com/google/tarpc/blob/master/tarpc/src/serde_transport.rs#L120

[SabatierBoris]

If we look on the CI log of the last PR merge : https://github.com/google/tarpc/actions/runs/7369173712/job/20061626669?pr=411 we don't find "serde_transport::tests::tcp"

But now we have it : https://github.com/google/tarpc/actions/runs/7459294314/job/20294984374?pr=414#step:10:34

[tikue]

Thanks!