build: fix build on cygwin
    
    Cygwin has getifaddrs(), but not AF_LINK, leading to:
    
    util/virstats.c: In function 'virNetInterfaceStats':
    util/virstats.c:138:41: error: 'AF_LINK' undeclared (first use in this function)
             if (ifa->ifa_addr->sa_family != AF_LINK)
    ...
    
    * src/util/virstats.c (virNetInterfaceStats): Only use getifaddrs
    if AF_LINK is present.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>