LibCore: Fix building with clang

Author: https://github.com/jcs
Commit: https://github.com/SerenityOS/serenity/commit/f6a8b1b69a2
Pull-request: https://github.com/SerenityOS/serenity/pull/1177

Libraries/LibCore/CNetworkJob.cpp

@@ -40,6 +40,14 @@ NetworkJob::~NetworkJob()
 {
 }
 
+void NetworkJob::start()
+{
+}
+
+void NetworkJob::shutdown()
+{
+}
+
 void NetworkJob::did_finish(NonnullRefPtr<NetworkResponse>&& response)
 {
     // NOTE: We protect ourselves here, since the on_finish callback may otherwise