selftests/landlock: Build without static libraries

The only (forced) static test binary doesn't depend on libcap.  Because
using -lcap on systems that don't have such static library would fail
(e.g. on Arch Linux), let's be more specific and require only dynamic
libcap linking.

Fixes: a5254052 ("selftests/landlock: Fix out-of-tree builds")
Cc: Anders Roxell <anders.roxell@linaro.org>
Cc: Guillaume Tucker <guillaume.tucker@collabora.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20221019200536.2771316-1-mic@digikod.net