bcachefs: add missing MODULE_DESCRIPTION()

Fix the 'make W=1' warning:
WARNING: modpost: missing MODULE_DESCRIPTION() in fs/bcachefs/mean_and_variance_test.o
Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/mean_and_variance_test.c

@@ -217,4 +217,5 @@ static struct kunit_suite mean_and_variance_test_suite = {
 kunit_test_suite(mean_and_variance_test_suite);
 
 MODULE_AUTHOR("Daniel B. Hill");
+MODULE_DESCRIPTION("bcachefs filesystem mean and variance unit tests");
 MODULE_LICENSE("GPL");