These fixes solve a compilation failure on FreeBSD:

util/virnetdevtap.c: In function 'virNetDevTapGetName':
util/virnetdevtap.c:56: warning: unused parameter 'tapfd' [-Wunused-parameter]
util/virnetdevtap.c:56: warning: unused parameter 'ifname' [-Wunused-parameter]

* src/util/virnetdevtap.c (virNetDevTapGetName): Add attributes
when TUNGETIFF is not present.
Signed-off-by: NEric Blake <eblake@redhat.com>