build: Don't ship GTEST headers in install target

A project that uses googletest(GTEST) shouldn't include googletest
headers with its artifacts in its install target.  These headers
simply are not needed by the consumers of the install target and
can cause conflicts with other projects that use googletest
themselves and this project's install target.  And they are just not
part of the expected glslang build artifacts.

It is likely that the addition of the googletest headers to the install
target was a simple oversight that happens as a result of adding
googletest as a CMake subdirectory.

For more information on how this causes conflicts with other projects,
please see: https://github.com/KhronosGroup/Vulkan-ValidationLayers/issues/821.