Resolve semicolon_if_nothing_returned clippy lint in test

    warning: consider adding a `;` to the last statement for consistent formatting
       --> tests/ffi/lib.rs:498:5
        |
    498 |     *out_param = "2020"
        |     ^^^^^^^^^^^^^^^^^^^ help: add a `;` here: `*out_param = "2020";`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#semicolon_if_nothing_returned
    note: the lint level is defined here
       --> tests/ffi/lib.rs:20:9
        |
    20  | #![deny(warnings)] // Check that expansion of `cxx::bridge` doesn't trigger warnings.
        |         ^^^^^^^^
        = note: `#[warn(clippy::semicolon_if_nothing_returned)]` implied by `#[warn(warnings)]`

tests/ffi/lib.rs

@@ -495,7 +495,7 @@ fn r_return_str(shared: &ffi::Shared) -> &str {
 
 fn r_return_str_via_out_param<'a>(shared: &'a ffi::Shared, out_param: &mut &'a str) {
     let _ = shared;
-    *out_param = "2020"
+    *out_param = "2020";
 }
 
 fn r_return_sliceu8(shared: &ffi::Shared) -> &[u8] {