build: fix unused variable in mingw
Bug introduced in commit 100b7a72a:
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: Eric Blake <eblake@redhat.com>