Fix compilation problem with 2.6 kernels

I had to apply this fix to be able to compile on 2.6 kernels (namely centos6 and debian6)

Without including `<sys/socket.h>` before `<linux/if_link.h>` compilation fails:
```
In file included from /usr/include/linux/if_link.h:5,
                 from nbwmon.c:4:
/usr/include/linux/netlink.h:34: error: expected specifier-qualifier-list before ‘sa_family_t’
make: *** [nbwmon] Error 1
```