kcsan: Add missing MODULE_DESCRIPTION() macro

Fix the warning reported by 'make C=1 W=1':
WARNING: modpost: missing MODULE_DESCRIPTION() in kernel/kcsan/kcsan_test.o

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>