dtc: Include stdlib.h in util.h

If used on its own, util.h needs stdlib.h for exit(), malloc() and
realloc().

Signed-off-by: Andrei Ziureaev <andrei.ziureaev@arm.com>
Message-Id: <20200721155900.9147-2-andrei.ziureaev@arm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

util.h

@@ -2,6 +2,7 @@
 #ifndef UTIL_H
 #define UTIL_H
 
+#include <stdlib.h>
 #include <stdarg.h>
 #include <stdbool.h>
 #include <getopt.h>