[llvm] [CMake] Fix DynamicLibraryTests exports symbol when plugins are enabled (PR #102941)

[Steven Wu via llvm-commits]

cachemeifyoucan wrote:

Second attempt to fix: https://lab.llvm.org/buildbot/#/builders/64/builds/643/steps/6/logs/FAIL__LLVM-Unit__DynamicLibraryTests_DynamicLibrar

I have no idea how that used to work on AIX builder though.

https://github.com/llvm/llvm-project/pull/102941