[llvm] llvm_gtest: Only install once (PR #96959)

[Will Dietz via llvm-commits]

dtzSiFive wrote:

It looks like this is installing the targets themselves into "lib" and their headers into the include directories.  What's being installed twice?

Looking at a build, I'm seeing various `libgtest*.a` in the "lib" output, with the headers in the "dev" / include path.

At least judging by "Installing: " lines I don't see anything installed twice, but maybe that's not a great oracle (or I'm not using the same install target(s))?

Can you elaborate a bit what's being fixed here?  Thanks!

https://github.com/llvm/llvm-project/pull/96959