[gn build] Port a349092

Author: llvmgnsyncbot

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

@@ -927,6 +927,7 @@ if (current_toolchain == default_toolchain) {
       "__utility/piecewise_construct.h",
       "__utility/priority_tag.h",
       "__utility/rel_ops.h",
+      "__utility/small_buffer.h",
       "__utility/swap.h",
       "__utility/to_underlying.h",
       "__utility/unreachable.h",