seccomp-util: make @known include @obsolete

@known is generated from syscall-list.txt, which generated from kernel
headers. So, some syscalls in @obsolete may not be listed in
syscall-list.txt.

src/shared/seccomp-util.c

@@ -928,6 +928,7 @@ const SyscallFilterSet syscall_filter_sets[_SYSCALL_FILTER_SET_MAX] = {
                 .name = "@known",
                 .help = "All known syscalls declared in the kernel",
                 .value =
+                "@obsolete\0"
 #include "syscall-list.h"
         },
 };