Change license of filenames.h to LGPL

[Florian Weimer]

* 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.

But then the implementation would need relicensing as well, wouldn't
it?

Having both under different licenses is just confusing.