gdbus-codegen-glibmm: bump to latest master

[martin-ejdestig]

Contains fixes and new API needed for first version of connectivity-manager.

Shortlog:

• Add missing return 0 for when Gio::DBus::NodeInfo::create_for_xml() fails
• Add usage_count() method to stub
• Use object id to check if object is already registered in stub
• Improve logging in register_object() of stub
• Add unregister_object() to stub and call it in destructor
• Make stub inherit from sigc::trackable
• Make stub follow rule of five
• Remove deprecated connect() method in stub
• Add missing return 0 to main() of integration test sources
• proxy.cpp.templ: call proxy's call_finish() in *_set_finish()

The commits related to making the stub handle deletion and unregistering from the bus properly are especially important since connectivity-manager creates/removes objects dynamically on the bus for Wi-Fi access points.

[martin-ejdestig]

Pushed a fix for the SRC_URI as well.