noop-renderer: Remove volatile and use compiler attribute

clang-13 complains about bitwise xor assigments like the following:

../libweston/noop-renderer.c:62:25: warning: variable 'unused' set but
not used [-Wunused-but-set-variable] volatile unsigned char unused = 0;

Use the __attribute__((unused)) instead.

Suggested-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

libweston/noop-renderer.c

@@ -58,7 +58,7 @@ noop_renderer_attach(struct weston_surface *es, struct weston_buffer *buffer)
 	struct wl_shm_buffer *shm_buffer;
 	uint8_t *data;
 	uint32_t size, i, width, height, stride;
-	volatile unsigned char unused = 0; /* volatile so it's not optimized out */
+	__attribute__((unused)) int unused = 0;
 
 	if (!buffer)
 		return;