Make sure valid utf-8 is passed to GtkTextView.

* src/gpacontext.c (gpa_context_get_diag): Change and convert if
needed.

On Windows we get the diagnostics in the native character set and thus
we would need to convert them before passing to other Gtk functions.
Given that this are only diagnostics we do a quick and safe
transliterate instead.

• Signed-off-by: Werner Koch <wk@gnupg.org>