[errorprone] Declare DirCache#version final

Enums values should be immutable, see
https://errorprone.info/bugpattern/ImmutableEnumChecker.

Change-Id: Ib0a358d3a5f1560ca73ec3153ca8088fe7a35eb6
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/dircache/DirCache.java

@@ -1033,7 +1033,7 @@ public class DirCache {
 		 */
 		DIRC_VERSION_PATHCOMPRESS(4);
 
-		private int version;
+		private final int version;
 
 		private DirCacheVersion(int versionCode) {
 			this.version = versionCode;