[ci] Allow platform references (flutter#6903)

Now that `platform` is no longer in-repo, it needs a dependency allowance.

Fixes tree breakage.

Author: stuartmorgan-g

script/configs/allowed_unpinned_deps.yaml

@@ -45,6 +45,7 @@
 - meta
 - mime
 - path
+- platform
 - shelf
 - shelf_static
 - source_gen