xbps-install: fflush before asking for confirmation

Closes #469

Author: paper42

bin/xbps-install/transaction.c

@@ -419,6 +419,8 @@ exec_transaction(struct xbps_handle *xhp, unsigned int maxcols, bool yes, bool d
 	 */
 	if ((rv = show_transaction_sizes(trans, maxcols)) != 0)
 		goto out;
+
+	fflush(stdout);
 	/*
 	 * Ask interactively (if -y not set).
 	 */