glib-genmarshal: close output file
Close output file to ensure all buffered output actually gets written.
Otherwise, glib-genmarshal output is sometimes empty (for example, when trying to build gdk-pixbuf on Windows, with Meson installed from .msi package).
argparse.FileType doesn't get closed automagically when the script exits: https://bugs.python.org/issue13824
Fixes #2341 (closed)