[gn build] Port e7a45c6

Author: llvmgnsyncbot

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

@@ -976,6 +976,8 @@ if (current_toolchain == default_toolchain) {
       "experimental/__simd/abi_tag.h",
       "experimental/__simd/aligned_tag.h",
       "experimental/__simd/declaration.h",
+      "experimental/__simd/internal_declaration.h",
+      "experimental/__simd/reference.h",
       "experimental/__simd/scalar.h",
       "experimental/__simd/simd.h",
       "experimental/__simd/simd_mask.h",