Add missing @Deprecated annotation

Noticed by error-prone (http://errorprone.info/bugpattern/DepAnn).

Change-Id: If8dcc2dd6d7cabffc95e10f8357ce31c12cf6b1c

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/FileTreeIterator.java

 	 * @deprecated use {@link #FileTreeIterator(FileTreeIterator, File, FS)}
 	 *             instead.
 	 */
+	@Deprecated
 	protected FileTreeIterator(final WorkingTreeIterator p, final File root,
 			FS fs) {
 		this(p, root, fs, DefaultFileModeStrategy.INSTANCE);