CI: Use Visual Studio 2019 instead of 2017

Visual Studio 2019 brings C11 support and is actually needed (or best
suited at least) if we are:

* Building with debugging code enabled (/Zc:preprocessor is required)
* Introspection works better on Visual Studio 2019 or later, also due to
  the preprocessor improvements.
* Pulling in bleeding edge Cairo (and possibly soon, GLib) already
  requires C11 support.

.gitlab-ci.yml

@@ -264,7 +264,7 @@ macos:
     paths:
       - "${CI_PROJECT_DIR}/_build/meson-logs"
 
-vs2017-x64:
+vs2019-x64:
   # TODO: Uncomment this when ready to merge.
   # rules:
   #   - if: $CI_PROJECT_NAMESPACE == "GNOME"