FindReadline.cmake typo preventing cmake config

Created by: iq2luc

Description CMake configuration: shared library libreadline.so not found due to a typo in FindReadline.cmake. Also see: https://github.com/OpenXRay/xray-16/pull/707#pullrequestreview-573280036

Steps to reproduce

1. Configure via CMake on Linux (Archlinux distro)
2. FindReadline.cmake cannot find libreadline.so

Suggested patch FixCMakeFindReadline.patch.txt