Adding CMake visibility policy setting
Adding CMake visibility policy setting

This policy setting will silence a warning when using with a visibility settings on targets. Due to the forced `cmake_minimum_version`, policy settings in CMakeLists calling this one (including the main CMakeLists) are lost, forcing the change to be made here.
google/googletest,fuchsia-mirror/third_party-googletest,xnagireddy/googletest,empiredan/googletest,fuchsia-mirror/third_party-googletest,google/googletest,antidotcb/googletest,fuchsia-mirror/third_party-googletest,gianricardo/googletest,xnagireddy/googletest,xnagireddy/googletest,jlanecox/googletest,fuchsia-mirror/third_party-googletest,jlanecox/googletest,antidotcb/googletest,antidotcb/googletest,jlanecox/googletest,gianricardo/googletest,empiredan/googletest,jlanecox/googletest,google/googletest,gianricardo/googletest,google/googletest,xnagireddy/googletest,antidotcb/googletest,empiredan/googletest