[gn build] Port f8350f1

Author: llvmgnsyncbot

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

@@ -490,6 +490,7 @@ if (current_toolchain == default_toolchain) {
       "__functional/weak_result_type.h",
       "__fwd/array.h",
       "__fwd/bit_reference.h",
+      "__fwd/byte.h",
       "__fwd/complex.h",
       "__fwd/deque.h",
       "__fwd/format.h",