meson: make source files including nspawn-settings.h depend on libseccomp

Since nspawn-settings.h includes seccomp.h, any file that includes
nspawn-settings.h should depend on libseccomp so the correct header path where
seccomp.h lives is added to the header search paths.

It's especially important for distros such as openSUSE where seccomp.h is not
shipped in /usr/include but /usr/include/libseccomp.

This patch is similar to 82384230.