seccomp: compel -- Add PTRACE_SECCOMP_GET_METADATA definition

We will use it to figure out if filter log target is used.
Metadata associated with seccomp filter is relatively new
feature which allows userspace to get and set it back.

Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>