weston-launch: Use /bin/sh for launching weston

https://bugs.freedesktop.org/show_bug.cgi?id=68739
diff --git a/src/weston-launch.c b/src/weston-launch.c
index e5301c7..8118d03 100644
--- a/src/weston-launch.c
+++ b/src/weston-launch.c
@@ -596,7 +596,7 @@
 	sigaddset(&mask, SIGINT);
 	sigprocmask(SIG_UNBLOCK, &mask, NULL);
 
-	child_argv[0] = wl->pw->pw_shell;
+	child_argv[0] = "/bin/sh";
 	child_argv[1] = "-l";
 	child_argv[2] = "-c";
 	child_argv[3] = BINDIR "/weston \"$@\"";