[flutter] Fix GN typo

Typo from https://dart-review.googlesource.com/c/sdk/+/164320 was not
caught on the Dart CQ.

dart_native_api.c (does not exist) should have been dart_api_dl.c.

Fix verified with Flutter build locally.

Change-Id: I7e37e4da27c02a2800ff36fd67e1be1575714c9c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/164560
Commit-Queue: Daco Harkes <[email protected]>
Reviewed-by: Clement Skau <[email protected]>

Author: dcharkes

runtime/BUILD.gn

@@ -207,8 +207,8 @@ source_set("dart_api") {
   public_configs = [ ":dart_public_config" ]
   sources = [
     "include/dart_api.h",
+    "include/dart_api_dl.c",
     "include/dart_api_dl.h",
-    "include/dart_native_api.c",
     "include/dart_native_api.h",
     "include/dart_tools_api.h",
     "include/dart_version.h",