lib: enable building KUnit in lib/

KUnit is a new unit testing framework for the kernel and when used is
built into the kernel as a part of it. Add KUnit to the lib Kconfig and
Makefile to allow it to be actually built.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Kees Cook <keescook@chromium.org>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>