[RFA/windows] Spurious "dll not found" error messages on x64-windows.

[Joel Brobecker]

> >gdb/ChangeLog:
> >
> >        * windows-nat.c: Add include of complaints.h.
> >        (handle_unload_dll): Change dll-not-found error into a complaint.
> 
> Looks good to me.  Thanks.

Thanks! Now checked in.

-- 
Joel