netdev: fix build on FreeBSD
    
    Commit ac3ed20 breaks build on FreeBSD with:
    
      CC       util/libvirt_util_la-virnetdev.lo
    util/virnetdev.c:2967:1: error: unused function 'virNetDevRDMAFeature' [-Werror,-Wunused-function]
    virNetDevRDMAFeature(const char *ifname,
    ^
    
    So hide virNetDevRDMAFeature function under the #ifdef 'SIOCETHTOOL'
    and 'HAVE_STRUCT_IFREQ' section.
    
    Pushed under the build breaker rule.