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>