opam: Fix typo in changelog

Signed-off-by: Euan Harris <euan.harris@citrix.com>
opam: Add build dependency on curl

We use the full release tarball so no sources have to be downloaded,
but the configure script fails if curl is not installed.

Signed-off-by: Euan Harris <euan.harris@citrix.com>
Merge pull request #509 from euanh/master

opam: Fix typo in changelog