[packages/firefox] filterout -Werror=format-security to fix build with gcc7

[Arkadiusz Miśkiewicz]

On Saturday 26 of August 2017, atler wrote:
> commit a132acc2832bf9003b9693a696c0cb6d96e44a5e
> Author: Jan Palus <atler at pld-linux.org>
> Date:   Sat Aug 26 22:25:55 2017 +0200
> 
>     filterout -Werror=format-security to fix build with gcc7

Why not fix these errors instead?

-- 
Arkadiusz Miśkiewicz, arekm / ( maven.pl | pld-linux.org )