Fix spotbugs warnings.

Author: nedtwigg

lib/src/main/java/com/diffplug/spotless/FileSignature.java

@@ -98,7 +98,9 @@ private FileSignature(final List<File> files) throws IOException {
 
 	/** A view of `FileSignature` which can be safely roundtripped. */
 	public static class RoundTrippable implements Serializable {
+		private static final long serialVersionUID = 1L;
 		private final List<File> files;
+		@SuppressFBWarnings("SE_TRANSIENT_FIELD_NOT_RESTORED")
 		private transient @Nullable FileSignature cached;
 
 		private RoundTrippable(List<File> files, FileSignature cached) {

lib/src/main/java/com/diffplug/spotless/JarState.java

@@ -39,6 +39,7 @@
 public final class JarState implements Serializable {
 	/** A lazily evaluated JarState, which becomes a set of files when serialized. */
 	public static class Promised implements Serializable {
+		private static final long serialVersionUID = 1L;
 		private final transient ThrowingEx.Supplier<JarState> supplier;
 		private FileSignature.RoundTrippable cached;
 

lib/src/main/java/com/diffplug/spotless/kotlin/DiktatStep.java

@@ -26,6 +26,7 @@
 
 /** Wraps up <a href="https://github.com/cqfn/diKTat">diktat</a> as a FormatterStep. */
 public class DiktatStep extends FormatterStepEqualityOnStateSerialization<DiktatStep.State> {
+	private static final long serialVersionUID = 1L;
 	private final JarState.Promised jarState;
 	private final boolean isScript;
 	private final @Nullable FileSignature.RoundTrippable config;