libsysdecode: fix build after 3d0a736796a9

TCP_USE_DDP is a kernel-only macro, so we must exclude it in
libsysdecode to avoid breaking the build.

Fixes: 3d0a736796a9 ("tcp: Add a new kernel-only TCP_USE_DDP [...]")
(cherry picked from commit 6b3db5d7793f57fa0020a78d61aae7ec318557cc)