[build] Fix Windows linker warning about unrecognized flag.

Change-Id: Iac7536450998f5f841e56ee57e60b36c4e71dabd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/332442
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

samples/ffi/http/BUILD.gn

@@ -5,5 +5,7 @@
 shared_library("fake_http") {
   sources = [ "lib/fake_http.cc" ]
   include_dirs = [ "." ]
-  ldflags = [ "-rdynamic" ]
+  if (!is_win) {
+    ldflags = [ "-rdynamic" ]
+  }
 }