Change license of filenames.h to LGPL

[Eli Zaretskii]

> From: DJ Delorie <dj@redhat.com>
> Cc: gcc-patches@gcc.gnu.org, sezeroz@gmail.com
> Date: Tue, 27 Sep 2016 13:52:10 -0400
> 
> 
> Most of the files in include/ are GPL, not LGPL.  Why is this one
> different?

Because Ozkan wants to use it in an otherwise LGPL package.