[gn build] Port 67b81e2

Author: llvmgnsyncbot

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -629,6 +629,7 @@ if (current_toolchain == default_toolchain) {
       "__memory/ranges_construct_at.h",
       "__memory/ranges_uninitialized_algorithms.h",
       "__memory/raw_storage_iterator.h",
+      "__memory/shared_count.h",
       "__memory/shared_ptr.h",
       "__memory/swap_allocator.h",
       "__memory/temp_value.h",