weston-launch: use exec to ensure signal delivery

Use exec to make sure the direct child process of weston-launch is
weston. This makes sure that signal delivery (SIGINT/SIGTERM) is
properly forwarded to weston.

Fixes ff3230952a ("weston-launch: Run weston in the user login shell")
Signed-off-by: Stefan Agner <stefan@agner.ch>

libweston/weston-launch.c

@@ -682,7 +682,7 @@ setup_session(struct weston_launch *wl, char **child_argv)
 	child_argv[0] = "/bin/sh";
 	child_argv[1] = "-l";
 	child_argv[2] = "-c";
-	child_argv[3] = BINDIR "/weston \"$@\"";
+	child_argv[3] = "exec " BINDIR "/weston \"$@\"";
 	child_argv[4] = "weston";
 	return 5;
 }