floss: Update topshim rebuild rule

Mismatched rebuild rule causes bt_topshim to be always rebuilt. This
fixes it so incremental build can be fast again.

Bug: 181590011
Tag: #floss
Test: manual - cargo build should not rebuild bt_topshim everytime

Change-Id: Icaebfd17aefbec2aa23355a1638cf184a7e92476