cmake: Bump min policy version

[arno-lunarg]

We are getting this error:

 CMake Error at external/Release/64/jsoncpp/build/install/lib/cmake/jsoncpp/jsoncppConfig.cmake:2 (cmake_policy):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.
Call Stack (most recent call first):
  CMakeLists.txt:32 (find_package)

Is bumping minimum to 3.5 ok?