vala: updated to 0.54.6

Vala 0.54.6
===========
* Various improvements and bug fixes:
  - codegen:
    + Allow boxing of non-external SimpleType structs
    + Cast given default-value of struct with possible member initializer
    + Clear existing length values when revisiting a slice expression
  - vala:
    + Allow unsafe assignment of integer to enum while reporting a notice
    + Non nullable enum types are simple types
    + Correctly replace "in" expression in pre-/postconditions of method

* Bindings:
  - gio-2.0: Add custom MemoryOutputStream.with_*data() wrappers

(adam)