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