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.

src/nspawn/nspawn-seccomp.c

@@ -6,16 +6,10 @@
 #include <sys/socket.h>
 #include <sys/types.h>
 
-#if HAVE_SECCOMP
-#include <seccomp.h>
-#endif
-
 #include "alloc-util.h"
 #include "log.h"
 #include "nspawn-seccomp.h"
-#if HAVE_SECCOMP
 #include "seccomp-util.h"
-#endif
 #include "string-util.h"
 #include "strv.h"