Update vala to 0.26.1

The hope is that backward compatibility will be sufficient for us
not to need to import every new stable version as a separate package.

Changes since vala024 was imported:

Vala 0.26.0
===========

* Bug fix and binding updates.

Vala 0.25.4
===========

* Add option --vapi-comments.
* Bug fixes and binding updates.

Vala 0.25.3
===========

* Bug fixes and binding updates.

Vala 0.25.2
===========

* Require and target GLib >= 2.24.
* Report warning for private struct fields.
* Replace webkit2gtk-3.0 bindings with webkit2gtk-4.0 bindings.
* Always build vapigen.
* Bug fixes and binding updates.

Vala 0.25.1
===========

* Support explicit interface method implementation.
* Support (unowned type)[] syntax.
* Support non-literal length in fixed-size arrays.
* Mark regular expression literals as stable.
* GIR parser updates.
* Add webkit2gtk-3.0 bindings.
* Add gstreamer-allocators-1.0 and gstreamer-riff-1.0 bindings.
* Bug fixes and binding updates.

(prlw1)