lkdtm: Do not depend on BLOCK and clean up headers

After the transition to kprobes, symbols are resolved at runtime. This
means there is no need to have all the Kconfig and header logic to
avoid build failures. This also paves the way to having arbitrary test
locations.

Reported-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Kees Cook <keescook@chromium.org>