dtc: Correct headers in util.c

Since util.c is used in programs other than full dtc, it shouldn't
include the full dtc.h, just util.h which has prototypes directly
relevant to it.  This patch makes the change, and also adds includes
of the necessary system headers which were previously included
indirectly by dtc.h.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

util.c

@@ -17,7 +17,12 @@
  *                                                                   USA
  */
 
-#include "dtc.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdarg.h>
+#include <string.h>
+
+#include "util.h"
 
 char *xstrdup(const char *s)
 {