UI: Update cmakelists to behave correctly on Linux
* The CMake Policy CMP0020 was introduced in 2.8.11 and allows CMake to automatically link against the necessary Qt5 libraries. In the linux chroots we are using CMake 2.8.10 and the policy is not recognized and thus was causing an error. This change only sets the policy if it will be understood by CMake.
* Remove the Qt5::WinMain link library as it is apparently not needed on Windows and is undefined in Linux so was causing a CMake error.