[libc++] Guard transitive include of `<locale>` with availability macro (#95686)

This is a follow-up to https://github.com/llvm/llvm-project/pull/80282.
The transitive includes of `<locale>` in `<vector>` were all guarded by
the availability macro -- the new include should also be guarded,
otherwise any users who compile with localization disabled will start
getting errors trying to include `<vector>`.

NOKEYCHECK=True
GitOrigin-RevId: 0cfdce854d588876bfca7030be868314e84c0e5b
1 file changed