include/uapi/linux/sockios.h: mark SIOCRTMSG unused

IOCTL SIOCRTMSG does nothing but return EINVAL.

So comment it as unused.

SIOCRTMSG is only used in:
* net/ipv4/af_inet.c
* include/uapi/linux/sockios.h

inet_ioctl calls ip_rt_ioctl.
ip_rt_ioctl only handles SIOCADDRT and SIOCDELRT and returns -EINVAL
otherwise.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>