[docker-sflow] limit privileged flag for sflow container (#16973)

#### Why I did it
HLD implementation: Container Hardening (sonic-net/SONiC#1364)
##### Work item tracking
- Microsoft ADO **(number only)**: 14807420
#### How I did it
Reduce linux capabilities in privileged flag

#### How to verify it
Run sflow sonic-mgmt tests
Check container's settings: Privileged is false and container only has default Linux caps, does not have extended caps.
```
admin@vlab-01:~$ docker inspect sflow | grep Privi
            "Privileged": false,


admin@vlab-01:~$ docker exec -it sflow bash
root@vlab-01:/# capsh --print
Current: cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap=ep
Bounding set =cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap
```

rules/docker-sflow.mk

@@ -30,7 +30,7 @@ SONIC_INSTALL_DOCKER_DBG_IMAGES += $(DOCKER_SFLOW_DBG)
 endif
 
 $(DOCKER_SFLOW)_CONTAINER_NAME = sflow
-$(DOCKER_SFLOW)_RUN_OPT += --privileged -t
+$(DOCKER_SFLOW)_RUN_OPT += -t
 $(DOCKER_SFLOW)_RUN_OPT += -v /etc/sonic:/etc/sonic:ro
 $(DOCKER_SFLOW)_RUN_OPT += -v /etc/timezone:/etc/timezone:ro 
 $(DOCKER_SFLOW)_RUN_OPT += -v /host/warmboot:/var/warmboot