Add GtkCss API to the introspection data

Some of the CSS API has been moved to a public namespace, so we need to
include it into the introspection data we build in order for people to
use it.

Fixes: #2230

gtk/meson.build

@@ -1104,6 +1104,8 @@ if build_gir
   gtk_introspection_sources = [
     gtk_public_headers,
     gtk_public_sources,
+    gtk_css_public_headers,
+    gtk_css_public_sources,
     gtk_deprecated_headers,
     gtk_deprecated_sources,
     a11y_headers,