Fix build when configuring with polkit0

Commit 2223ea98 removed the only use of 'server' param in
remoteDispatchAuthPolkit().  Mark the parameter with ATTRIBUTE_UNUSED
to fix the build when configuring with polkit0.

daemon/remote.c

@@ -2653,7 +2653,7 @@ authdeny:
 }
 #elif HAVE_POLKIT0
 static int
-remoteDispatchAuthPolkit(virNetServerPtr server,
+remoteDispatchAuthPolkit(virNetServerPtr server ATTRIBUTE_UNUSED,
                          virNetServerClientPtr client,
                          virNetMessagePtr msg ATTRIBUTE_UNUSED,
                          virNetMessageErrorPtr rerr,