plugins: Update stale comment

"plugin_mask" was renamed as "event_mask" in commit c006147122
("plugins: create CPUPluginState and migrate plugin_mask").

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240427155714.53669-3-philmd@linaro.org>
1 file changed