ipv4: Exposing __ip_sock_set_tos() in ip.h

Making the static function __ip_sock_set_tos() from net/ipv4/ip_sockglue.c
accessible by declaring it in include/net/ip.h
The reason for doing this is to use this function to set IP_TOS value in
mptcp_setsockopt() without the lock.

Signed-off-by: Poorva Sonparote <psonparo@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>