util: re-add conditional for ifi_iqdrops field for macOS

util: re-add conditional for ifi_iqdrops field for macOS

The conditional was removed in

  commit ebbf8ebe4fa6f9d43b40673f0f2dad6bf50e2085
  Author: Ján Tomko <jtomko@redhat.com>
  Date:   Tue Sep 1 22:56:37 2020 +0200

    util: virnetdevtap: stats: fix txdrop on FreeBSD

That commit was correct about this no longer being required for FreeBSD,
but missed that the code is also built on macOS.

Rather than testing for this field in meson though, we can simply use
a platform conditional test in the code.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>