Fix regression with building dependencies

This fixes an issue where cargo refinedrust would fail with obscure errors when building some dependencies like libclang-sys on Ubuntu and possibly other Linux systems.

rr_frontend/refinedrust_frontend/tests/deps_build/src/lib.rs

+/// This is empty to test the build of the libc dependency, as a guard against regressions of https://gitlab.mpi-sws.org/lgaeher/refinedrust-dev/-/merge_requests/50.
+
+fn dummy() {}