selftests: enforce local header dependency in lib.mk

Add local header dependency in lib.mk. This enforces the dependency
blindly even when a test doesn't include the file, with the benefit
of a simpler common logic without requiring individual tests to have
special rule for it.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>