[CMake] Fix the setting of LIBCXX_HEADER_DIR

Reviewers: phosek

Subscribers: mgorny, christof, ldionne, cfe-commits

Differential Revision: https://reviews.llvm.org/D49629

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337727 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed