meson: qt: add missing csdbuttonmodel sources

This was causing a link failure:

/usr/include/x86_64-linux-gnu/qt5/QtCore/qmetatype.h:1590: undefined reference to `CSDButton::staticMetaObject'

It was recently added by 99ee50c5.

modules/gui/qt/meson.build

@@ -115,6 +115,7 @@ moc_headers = files(
     'util/cliplistmodel.hpp',
     'util/color_scheme_model.hpp',
     'util/color_svg_image_provider.hpp',
+    'util/csdbuttonmodel.hpp',
     'util/imageluminanceextractor.hpp',
     'util/i18n.hpp',
     'util/keyhelper.hpp',
@@ -398,6 +399,8 @@ some_sources = files(
     'util/color_svg_image_provider.hpp',
     'util/covergenerator.cpp',
     'util/covergenerator.hpp',
+    'util/csdbuttonmodel.cpp',
+    'util/csdbuttonmodel.hpp',
     'util/imageluminanceextractor.cpp',
     'util/imageluminanceextractor.hpp',
     'util/imagehelper.cpp',