[infra] //build/bootdata.gni -> //build/config/fuchsia/bootdata.gni

This new Fuchsia GN infrastructure file actually landed by a slightly
different name, so update the reference to it.

Change-Id: I52ba2bc91eb3125c938added87d8ad3c864371d4
Reviewed-on: https://dart-review.googlesource.com/16302
Reviewed-by: Zach Anderson <zra@google.com>

BUILD.gn

@@ -124,7 +124,7 @@ group("samples") {
 # by the Fuchsia build, add the GN build argument:
 #   extra_bootdata = [ "//third_party/dart:dart_test_bootfs" ]
 if (is_fuchsia) {
-  import("//build/bootdata.gni")
+  import("//build/config/fuchsia/bootdata.gni")
 
   action("generate_dart_test_manifest") {
     testonly = true