[gn build] Port 2a8f9a5

Author: llvmgnsyncbot

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

@@ -456,6 +456,7 @@ if (current_toolchain == default_toolchain) {
       "__utility/swap.h",
       "__utility/to_underlying.h",
       "__utility/transaction.h",
+      "__utility/unreachable.h",
       "__variant/monostate.h",
       "algorithm",
       "any",