vala: update to 0.48.8.

Vala 0.48.8
===========
 * Various improvements and bug fixes:
  - codegen:
    + Handle "gtk_param_spec_expression" in get_param_spec()
    + Properly compare string if binary-expression contains string-literal
    + Inline allocated and constant arrays are never NULL
    + Check param for null before using it
    + Properly destroy elements of an inline struct array [#365]
  - vala:
    + Avoid subsequent critical caused by invalid array type for constant
    + Improve error handling of nested initializer lists
    + Don't allow "resize" invocation on variable with unowned array type [#928]
    + Property must be writable when invoking ReturnsModifiedPointer method [#1023]
    + Recursive check of assignment target to recognize constants [#944]
    + Return copy of constant type in get_value_type_for_symbol()
    + Handle parameter initializer without value-type
    + Fully qualify member-access when using-directive was used to resolve [#57]
    + Tranform instance member-access to a static one if possible [#270]
    + Improve check of expression passed to yield
    + Allow prototype access to signal with emitter
    + Check error-type accessibility of creation methods and delegates
  - girparser: Warn about conflicting signals and properties
  - girparser: Skip 'docsection' elements in <namespace>
  - girwriter:
    + Add c:identifier-prefixes to <namespace>
    + Write c:type="void" when name="none"
    + Write c:symbol-prefix on supported elements
    + Don't include symbols outside of a namespace [#241]
    + Use appropriate get_ccode_* functions
  - libvaladoc: Support exporting the `class` modifier on methods and fields
  - Disable deprecation warnings when checking bindings
  - tests: Fix "methods/extern" test with -Werror=address and gcc 5.4.0

 * Bindings:
   - glib-2.0: Add AtomicUint, an alternation of AtomicInt for uint [#915]
   - gstreamer: Update from 1.17.2+ git master
   - gtk+-3.0: Update to 3.24.21+ffe2df7b
   - gtk+-3.0: Resolve conflicts of signals with properties
   - gtk4: Resolve conflict of DropTarget.drop signal with property [#1033]
   - gtk4: Improve Gtk.ParamSpecExpression binding
   - gtk4: Fix binding of CClosureExpression ctor [#1024]
   - gtk4,gtk4-unix-print: Update to 3.98.5+23dcf36e
   - webkit2gtk-4.0: Update to 2.29.4


(wiz)