Tag InterestingLimit enum with target_os

[CriesofCarrots]

Problem

#34149 added some target_os = "linux" handling, but the accompanying enum is not tagged, leading to clippy complaints on other OSes, like mac:

warning: variants `Recommend` and `QueryOnly` are never constructed
   --> core/src/system_monitor_service.rs:397:5
    |
396 | enum InterestingLimit {
    |      ---------------- variants in this enum
397 |     Recommend(i64),
    |     ^^^^^^^^^
398 |     QueryOnly,
    |     ^^^^^^^^^
    |
    = note: `#[warn(dead_code)]` on by default

warning: `solana-core` (lib) generated 1 warning

Summary of Changes

Tag the enum

[t-nelson]

Only linux has interesting limits

true. thanks!

[mergify]

automerge label removed due to a CI failure