plug-ins: use g_mkdir() for generic cross-platform code.

This fixes the following error when building for Windows:

> error: too many arguments to function ‘mkdir’

On Windows, mkdir() is deprecated, it is an alias for _mkdir() which
doesn't have a mode parameter. The GLib version duplicates the POSIX
signature and is cross-platform (mode is simply ignored on Windows).

plug-ins/script-fu/ftx/ftx.c

@@ -274,7 +274,7 @@ pointer foreign_mkdir(scheme *sc, pointer args)
   else
     mode = 0777;
 
-  retcode = mkdir(dirname, (mode_t)mode);
+  retcode = g_mkdir(dirname, (mode_t)mode);
   if (retcode == 0)
     return sc->T;
   else