Allowlist fs_msg by package

Fixes a build breakage that had a new fs_msg-pkg dependency added. This
would either have to be allowlisted by name, or we'd have to allowlist
the whole package.

Allowlisting the whole package is safer with allowlist v2.

Test: Presubmits
Change-Id: Id3b5c39c60519e966b53ec4fa228c3a79e68fa7a