Sort the list of files being processed by glib-mkenums

We should ensure a stable order when processing the files, regardless of
the order they were submitted on the command line, to increase the
chances of a reproducible build.

The old Perl-based version of glib-mkenums was fixed in commit 8686e430
to do the same.

https://bugzilla.gnome.org/show_bug.cgi?id=691436

gobject/glib-mkenums.in

@@ -683,7 +683,7 @@ def process_file(curfilename):
                 prod = replace_specials(prod)
                 write_output(prod)
 
-for fname in options.args:
+for fname in sorted(options.args):
     process_file(fname)
 
 if len(ftail) > 0: