Fix findbugs warnings.

Author: nedtwigg

lib-extra/src/main/java/com/diffplug/spotless/extra/P2Mirror.java

@@ -15,6 +15,9 @@
  */
 package com.diffplug.spotless.extra;
 
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
+@SuppressFBWarnings("UWF_UNWRITTEN_FIELD")
 public class P2Mirror {
 
 	private String prefix;