codegen: Only set "sentinel" CCode attribute for node if requested
codegen: Only set "sentinel" CCode attribute for node if requested

This should restrict this being set only on Method nodes.

frida/vala,GNOME/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,frida/vala,GNOME/vala,frida/vala