Fix for 'linux/inet_diag.h: No such file or directory'

Hello,

This PR contains a fix for compilation failure with old Linux kernels (including 2.6.9 in RHEL 4).

The problem : 

    tcpconns.c:77:30: linux/inet_diag.h : No such file or directory

And other errors like NETLINK_INET_DIAG undeclared... because of this missing header.

Note : this is a new version of PR #215.

Regards,
Yves