[build] Fix MSVC build.

Broken in a2c47adfba.

Change-Id: I67a7252ab47ff2b5fdd01cc107e83ed0f53c86e9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/327662
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

third_party/binaryen/BUILD.gn

@@ -18,7 +18,7 @@ config("binaryen_flags") {
   }
 
   # Use legacy encoding for WasmGC instructions
-  cflags += [ "-DUSE_LEGACY_GC_ENCODINGS" ]
+  defines = [ "USE_LEGACY_GC_ENCODINGS" ]
 
   if (is_win) {
     libs = [ "shell32.lib" ]