dnd: Use %zd in the format string for size_t types

len i size_t, so is the result of the sizeof operator.
diff --git a/clients/dnd.c b/clients/dnd.c
index 4c070ff..4aad070 100644
--- a/clients/dnd.c
+++ b/clients/dnd.c
@@ -517,7 +517,7 @@
 	if (len == 0) {
 		return;
 	} else if (len != sizeof *message) {
-		fprintf(stderr, "odd message length %ld, expected %ld\n",
+		fprintf(stderr, "odd message length %zu, expected %zu\n",
 			len, sizeof *message);
 		return;
 	}