xwayland: Use _exit() when exec() fails

Similar to 3b64d6 but for when we fork the X server.
diff --git a/src/xwayland/launcher.c b/src/xwayland/launcher.c
index a8ad478..7048131 100644
--- a/src/xwayland/launcher.c
+++ b/src/xwayland/launcher.c
@@ -72,7 +72,7 @@
 			  "-terminate",
 			  NULL) < 0)
 			weston_log("exec failed: %m\n");
-		exit(-1);
+		_exit(EXIT_FAILURE);
 
 	default:
 		weston_log("forked X server, pid %d\n", wxs->process.pid);