[CK_TILE] Fix Windows build in FMHA head grouping
## Motivation
This is a follow-up fix for [PR
#5018](https://github.com/ROCm/rocm-libraries/pull/5018).
[PR #5018](https://github.com/ROCm/rocm-libraries/pull/5018) added
LLC-aware FMHA head grouping / head-major scheduling on RDNA, but it
also introduced Linux-only code paths, including `<dirent.h>`, which
break Windows builds. This change fixes that by guarding the
Linux-specific LLC probing logic so non-Linux platforms can still build
correctly.
## Technical Details
- Guard `<dirent.h>` with `#ifdef __linux__`
- Guard KFD sysfs traversal logic with `#if defined(__linux__)`
- On non-Linux platforms, return `0` from
`get_kfd_sysfs_llc_cache_bytes()`
- Preserve existing fallback behavior through:
- `CK_TILE_FMHA_LLC_CACHE_MB`
- arch-based default LLC sizes
- no head grouping when no LLC size can be resolved
## Test Plan
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## Test Result
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## Submission Checklist
- [ ] Look over the contributing guidelines at
https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.
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