util: virnetdevtap: stats: fix txdrop on FreeBSD

util: virnetdevtap: stats: fix txdrop on FreeBSD

For older FreeBSD, we needed an ifdef guard to use
if_data.ifi_oqdrops, which was introduced by:

commit 61bbdbb94ce3e2f5e969c9bddb443427db07bf61
    Implement interface stats for BSD

But when we dropped the check because we deprecated
building on FreeBSD-10 in:

commit 83131d9714db7ee77ab220186b6b0d8b6c22b09e
    configure: drop check for unsupported FreeBSD

We started building the wrong side of the ifdef.

Signed-off-by: Ján Tomko <jtomko@redhat.com>
Fixes: 83131d9714db7ee77ab220186b6b0d8b6c22b09e
Reviewed-by: Roman Bogorodskiy <bogorodskiy@gmail.com>