Bug#846437: linux-atm: add libfl-dev to Build-Depends

[Helmut Grohne <helmut@subdivi.de>]

Source: linux-atm
Severity: important
User: helmutg@debian.org
Usertags: libfldep

linux-atm will soon fail to build from source, beause flex drops its
dependency on libfl-dev. Since linux-atm uses parts of libfl-dev
(e.g. libl.a, libfl.a or FlexLexer.h), it should add libfl-dev to its
Build-Depends. This change was previously announced[1] to
debian-devel in accordance with DevRef 7.1.1. Please add the missing
dependency.

Helmut

[1] https://lists.debian.org/debian-devel/2016/03/msg00162.html