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

Due to this specific change: d0b6a10#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

meson.build

@@ -2732,7 +2732,8 @@ exe = executable('systemd-nspawn',
                  link_with : [libcore_shared,
                               libnspawn_core,
                               libshared],
-                 dependencies : [libblkid],
+                 dependencies : [libblkid,
+                                 libseccomp],
                  install_rpath : rootlibexecdir,
                  install : true)
 public_programs += exe