tar: pacify GCC 5.1 -Wformat-signedness

* lib/wordsplit.c (struct wordsplit_node.flags):
Now unsigned, so that 'printf ("%x", p->flags)' doesn't provoke GCC.
* src/incremen.c (read_num, dumpdir_ok):
Don't printf an int with %x or %o.

lib/wordsplit.c

@@ -221,7 +221,7 @@ struct wordsplit_node
 {
   struct wordsplit_node *prev;	/* Previous element */
   struct wordsplit_node *next;	/* Next element */
-  int flags;			/* Node flags */
+  unsigned flags;		/* Node flags */
   union
   {
     struct