[libc++] Unbreak Apple buildbots

These buildbots are using the deprecated target name install-libcxx-headers
instead of the more up to date install-cxx-headers, so I need to add an
install-libcxx-headers-stripped target to satisfy them.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@320201 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed