Fix Rust shared polyglot test case for cross-compilation
The crate-type in this case should have been `cdylib` since it's
linking via C, rather than the default `dylib` that is meant for
linking via `rustc`.
project('rust and c polyglot executable', 'c', 'rust')
-l = library('stuff', 'stuff.rs', install : true)
+l = library('stuff', 'stuff.rs', rust_crate_type: 'cdylib', install : true)
e = executable('prog', 'prog.c', link_with : l, install : true)
test('polyglottest', e)
