Bug#846436: librsl: add libfl-dev to Build-Depends

[Helmut Grohne]

Source: librsl
Severity: important
User: helmutg at debian.org
Usertags: libfldep

librsl will soon fail to build from source, beause flex drops its
dependency on libfl-dev. Since librsl 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