Sample.Gtk: add SourceGen as ProjectReference

Added this to ensure that SourceGen.dll is created before
building the Controls.Sample.Gtk. We have to do this since
sometimes when building the Sample.Gtk project, the error
below is shown. It's a random error that happens when
building the Microsoft.Maui.Gtk.slnf but it doesn't happen
in each build attempt.

```
CSC : error CS0006: Metadata file '/home/runner/work/maui/maui/.nuspec/Microsoft.Maui.Controls.SourceGen.dll' could not be found [/home/runner/work/maui/maui/src/Controls/samples/Controls.Sample.Gtk/Controls.Sample.Gtk.csproj]
    0 Warning(s)
    1 Error(s)
```

.github/workflows/build-gtk.yml

@@ -36,6 +36,7 @@ jobs:
             dotnet workload install gtk --skip-manifest-update
       - name: Build MAUI
         run: |
+            dotnet build src/Controls/src/SourceGen/Controls.SourceGen.csproj
             dotnet build Microsoft.Maui.Gtk.slnf
 
   dotnet-format: