Fix build with polkit0

I missed adding virNetServerGetDBusConn() to libvirtd_private.syms
in commit b8adfcc6, which didn't cause a problem in 0.9.6 but
results in this build error in 0.9.7

libvirtd-remote.o: In function `remoteDispatchAuthPolkit':
remote.c:(.text+0x188dd): undefined reference to `virNetServerGetDBusConn'

src/libvirt_private.syms

@@ -1261,6 +1261,7 @@ virNetServerAddSignalHandler;
 virNetServerAutoShutdown;
 virNetServerClose;
 virNetServerFree;
+virNetServerGetDBusConn;
 virNetServerIsPrivileged;
 virNetServerNew;
 virNetServerQuit;