headless: fix uninitialized variable

`no_outputs` is declared on the stack and left uninitialized if no
weston option changing its value is provided.

Signed-off-by: Guillaume Champagne <champagne.guillaume.c@gmail.com>

compositor/main.c

@@ -2550,7 +2550,7 @@ load_headless_backend(struct weston_compositor *c,
 	const struct weston_windowed_output_api *api;
 	struct weston_headless_backend_config config = {{ 0, }};
 	struct weston_config_section *section;
-	bool no_outputs;
+	bool no_outputs = false;
 	int ret = 0;
 	char *transform = NULL;