selftests: rust: config: disable GCC_PLUGINS

CONFIG_RUST depends on !CONFIG_GCC_PLUGINS. Disable CONFIG_GCC_PLUGINS
in rust/config file to make sure it doesn't get enabled.
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/rust/config

+# CONFIG_GCC_PLUGINS is not set
 CONFIG_RUST=y
 CONFIG_SAMPLES=y
 CONFIG_SAMPLES_RUST=y