[CMake] Move cxx-headers to RUNTIME_DISTRIBUTION_COMPONENTS in Apple-stage2.cmake

We build libcxx using LLVM_ENABLE_RUNTIMES during Stage2, which requires
cxx-headers to be part of LLVM_RUNTIME_DISTRIBUTION_COMPONENTS instead
of LLVM_DISTRIBUTION_COMPONENTS.

rdar://99028431

Differential Revision: https://reviews.llvm.org/D132488