Fixes BUILD.gn if is_fuchsia (legacy embedder) and is_debug (flutter/engine#25858)

Current recommended build methods may not trigger this issue, but
while trying different options, building flutter for the first time, I
noticed a build error because `defines` was assigned twice.

If building with both options is ever supported, I think this
fixes the error.

engine/src/flutter/BUILD.gn

@@ -21,16 +21,18 @@ config("config") {
     }
   }
 
+  defines = []
+
   # This define is transitional and will be removed after the embedder API
   # transition is complete.
   #
   # TODO(bugs.fuchsia.dev/54041): Remove when no longer necessary.
   if (is_fuchsia && flutter_enable_legacy_fuchsia_embedder) {
-    defines = [ "LEGACY_FUCHSIA_EMBEDDER" ]
+    defines += [ "LEGACY_FUCHSIA_EMBEDDER" ]
   }
 
   if (is_debug) {
-    defines = [ "FLUTTER_ENABLE_DIFF_CONTEXT" ]
+    defines += [ "FLUTTER_ENABLE_DIFF_CONTEXT" ]
   }
 }