string: kunit: add missing MODULE_DESCRIPTION() macros

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

Add the missing invocation of the MODULE_DESCRIPTION() macro.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://lore.kernel.org/r/20240531-md-lib-string-v1-1-2738cf057d94@quicinc.com


Signed-off-by: Kees Cook <kees@kernel.org>