sys_linux: allow setsockopt(SOL_IP, IP_TOS) in seccomp

sys_linux: allow setsockopt(SOL_IP, IP_TOS) in seccomp

This system call is required by the DSCP marking feature introduced in commit
6a5665ca5877 ("conf: add dscp directive").

Before this change, enabling seccomp filtering (chronyd -F 1) and specifying a
custom DSCP value in the configuration (for example "dscp 46") caused the
process to be killed by seccomp due to IP_TOS not being allowed by the filter.

Tested before and after the change on Ubuntu 21.04, kernel 5.11.0-13-generic.
IP_TOS is available since Linux 1.0, so I didn't add any ifdefs for it.

Signed-off-by: Foster Snowhill <forst@forstwoof.ru>