selftests: rust: config: add trailing newline

If adding multiple config files to the merge_config.sh script and
rust/config is the fist one, then the last config fragment in this file
and the first config fragment in the second file won't be set, since
there isn't a newline in this file, so those two fragements end up at
the same row like:
CONFIG_SAMPLE_RUST_PRINT=mCONFIG_FRAGMENT=y

And non of those will be enabled when running 'olddefconfig' after.

Fixing the issue by adding a newline to the file.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>