GIR warnings

## Describe the bug
Building the latest released granite I see these warnings in the build output
```
../lib/Services/Application.vala:21.1-21.26: warning: Namespace Services does not have a GIR namespace and version annotation
namespace Granite.Services {
^^^^^^^^^^^^^^^^^^^^^^^^^^
../lib/GtkPatch/AboutDialog.vala:23.1-23.54: warning: Namespace GtkPatch does not have a GIR namespace and version annotation
public class Granite.GtkPatch.AboutDialog : Gtk.Dialog {
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
../lib/Drawing/BufferSurface.vala:25.1-25.25: warning: Namespace Drawing does not have a GIR namespace and version annotation
namespace Granite.Drawing {
^^^^^^^^^^^^^^^^^^^^^^^^^
../lib/Widgets/AboutDialog.vala:22.1-22.25: warning: Namespace Widgets does not have a GIR namespace and version annotation
namespace Granite.Widgets {
```

## Platform Information
NixOS 20.30 (development).

Please check what applies:
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- [ ] I'm using the latest version from git that I've manually compiled
- [x] I'm using the latest released stable version