Commit ac3ed208 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.