Fix spotbugs.

Author: nedtwigg

lib-extra/src/groovy/java/com/diffplug/spotless/extra/glue/groovy/GrEclipseFormatterStepImpl.java

@@ -103,7 +103,7 @@ public String format(String raw) throws Exception {
 	/**
 	 * Eclipse Groovy formatter does not signal problems by its return value, but by logging errors.
 	 */
-	private static class GroovyErrorListener implements ILogListener, IGroovyLogger {
+	private static final class GroovyErrorListener implements ILogListener, IGroovyLogger {
 		private final List<String> errors;
 
 		public GroovyErrorListener() {