os: Ensure <dix-config.h> is included in strndup.c

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>

os/strndup.c

@@ -27,6 +27,10 @@
  * SUCH DAMAGE.
  */
 
+#ifdef HAVE_DIX_CONFIG_H
+#include <dix-config.h>
+#endif
+
 #include <stddef.h>
 #include <stdlib.h>
 #include <string.h>