[CMake] Use correct variable as header install prefix

This variable is already set in CMakeLists.txt but it wasn't used
which means that the headers get installed into a wrong location
when the per target runtime directory option is being used.

Differential Revision: https://reviews.llvm.org/D49345

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337118 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed