build: mark conditionally unused variables

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>

src/util/virnetdevtap.c

@@ -53,7 +53,7 @@
  * Returns 0 if the interface name is successfully queried, -1 otherwise
  */
 int
-virNetDevTapGetName(int tapfd, char **ifname)
+virNetDevTapGetName(int tapfd ATTRIBUTE_UNUSED, char **ifname ATTRIBUTE_UNUSED)
 {
 #ifdef TUNGETIFF
     struct ifreq ifr;