build: fix unused variable in mingw

Bug introduced in commit 100b7a72:

util/virnetdevbridge.c: In function 'virNetDevBridgePortSetLearning':
util/virnetdevbridge.c:359:38: error: unused parameter 'enable' [-Werror=unused-parameter]
                                bool enable)
                                      ^

* src/util/virnetdevbridge.c (virNetDevBridgePortSetLearning): Mark
unused variable.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/util/virnetdevbridge.c

@@ -356,7 +356,7 @@ virNetDevBridgePortGetLearning(const char *brname ATTRIBUTE_UNUSED,
 int
 virNetDevBridgePortSetLearning(const char *brname ATTRIBUTE_UNUSED,
                                const char *ifname ATTRIBUTE_UNUSED,
-                               bool enable)
+                               bool enable ATTRIBUTE_UNUSED)
 {
     virReportSystemError(ENOSYS, "%s",
                          _("Unable to set bridge port learning on this platform"));