meson: add libseccomp as a nspawn dep (#12067)

Due to this specific change: d0b6a10c#diff-0203416587516c224c8fcfe8129e7caeR8,
systemd-nspawn uses libseccomp now if it is available. We we need to pass -I/usr/include
/libseccomp (or wherever seccomp.h is located) when compiling systemd-nspawn because
nspawn-settings.h does #include <seccomp.h>.

Fixes: #12060