[libc++] Revert to using PUBLIC instead of PRIVATE when linking system libs

It turns out the benchmarks need to link against those libraries
explicitly too, so CMake's propagation of PUBLIC dependencies is
used.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@373506 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed