tree-wide: don't ifdef seccomp-util.h, drop seccomp.h inclusion everywhere

tree-wide: don't ifdef seccomp-util.h, drop seccomp.h inclusion everywhere

seccomp-util.h doesn't need ifdeffing, hence don't. It has worked since
quite a while with HAVE_SECCOMP is off, hence use it everywhere.

Also drop explicit seccomp.h inclusion everywhere (which needs
HAVE_SECCOMP ifdeffery everywhere). seccomp-util.h includes it anyway,
automatically, which we can just rely on, and it deals with HAVE_SECCOMP
at one central place.