Bug#1111811: haskell-erf: Please copy the BSD-3-clause to copyright file

[Bastian Germann]

Source: haskell-erf
Version: 2.0.0.0-19
Severity: normal

For the BSD3 string in Cabal metadata, Cabal refers to
https://opensource.org/license/bsd-3-clause which is slightly different
from the original BSD-3-clause in the common-licenses. That file is also
discouraged from being referred by Policy to for exactly this reason.

Please copy the right license text to the copyright file.