lib: bitmap: add missing MODULE_DESCRIPTION() macros

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/find_bit_benchmark.o
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/cpumask_kunit.o
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/test_bitmap.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>