build: avoid unused symbol

Commit f36309d6 added an export with no matching implementation;
probably a misspelling of an earlier version of the final addition
of virNetworkObjSetDefTransient.

* src/libvirt_private.syms (network_conf.h): Drop bogus
virNetworkSetDefTransient.

src/libvirt_private.syms

@@ -869,7 +869,6 @@ virNetworkSaveConfig;
 virNetworkSaveStatus;
 virNetworkSetBridgeMacAddr;
 virNetworkSetBridgeName;
-virNetworkSetDefTransient;
 virPortGroupFindByName;