kunit: add missing MODULE_DESCRIPTION() macros to lib/*.c

make allmodconfig && make W=1 C=1 reports for lib/*kunit:
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/bitfield_kunit.o
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/checksum_kunit.o
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/cmdline_kunit.o
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/is_signed_type_kunit.o
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/overflow_kunit.o
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/stackinit_kunit.o

Add the missing invocations of the MODULE_DESCRIPTION() macro.

Link: https://lkml.kernel.org/r/20240601-md-lib-kunit-tests-v1-1-4493fe0032b9@quicinc.com


Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>