[gardening] Fix SDK build

https://dart-review.googlesource.com/c/sdk/+/365581 rolled Binaryan.
This caused the following error:
../../third_party/binaryen/src/src/support/small_vector.h:39:32: error: unknown warning group '-Wmaybe-uninitialized', ignored [-Werror,-Wunknown-warning-option]

Prevent the error by adding `-Wno-unknown-warning-option`.

TEST=local SDK build on arm64 MacOS

Change-Id: I0c1315bdc0a94db3158f657de9d155d6c796b3c8
Cq-Include-Trybots: luci.dart.try:dart-sdk-linux-arm64-try,dart-sdk-linux-riscv64-try,dart-sdk-linux-try,dart-sdk-mac-arm64-try,dart-sdk-mac-try,dart-sdk-win-arm64-try,dart-sdk-win-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/365583
Commit-Queue: Daco Harkes <[email protected]>
Reviewed-by: Martin Kustermann <[email protected]>

Author: dcharkes

third_party/binaryen/BUILD.gn

@@ -24,6 +24,7 @@ config("binaryen_flags") {
         "-Wno-header-hygiene",
         "-Wno-unused-private-field",
         "-Wno-inconsistent-missing-override",
+        "-Wno-unknown-warning-option",
       ]
     } else {
       # GCC