libarchive: Fix include order in xxhash.c

We need to include `archive_platform.h` before any system headers in
order to ensure that `_WIN32_WINNT` is defined early enough.

Utilities/cmlibarchive/libarchive/xxhash.c

@@ -29,12 +29,13 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 You can contact the author at :
 - xxHash source repository : http://code.google.com/p/xxhash/
 */
-#include <stdlib.h>
-#include <string.h>
 
 #include "archive_platform.h"
 #include "archive_xxhash.h"
 
+#include <stdlib.h>
+#include <string.h>
+
 #ifdef HAVE_LIBLZ4
 
 /***************************************