[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
diff --git a/include/CMakeLists.txt b/include/CMakeLists.txt
index 6b1383f..b98e092 100644
--- a/include/CMakeLists.txt
+++ b/include/CMakeLists.txt
@@ -63,6 +63,7 @@
 
     add_custom_target(libcxx-headers)
     add_custom_target(install-libcxx-headers DEPENDS install-cxx-headers)
+    add_custom_target(install-libcxx-headers-stripped DEPENDS install-cxx-headers-stripped)
   endif()
 
 endif()