backend-x11: fix pixman output state creation

Pixman output state creation requires a valid pixel format to be set for
weston_output_update_capture_info().

Fixes: c67773bc5c ("pixman-renderer: use pixel_format_info instead of pixman_format_code_t")
Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>

libweston/backend-x11/x11.c

@@ -1040,6 +1040,7 @@ x11_output_enable(struct weston_output *base)
 				.width = mode->width,
 				.height = mode->height
 			},
+			.format = pixel_format_get_info_by_pixman(PIXMAN_x8r8g8b8)
 		};
 		if (x11_output_init_shm(b, output,
 					mode->width, mode->height) < 0) {