nanostack: fix inclusion of kvstore

[0xc0170]

Summary of changes

Protect kvstore headers as well as they should not be included if kvstore is not enabled
in nanostack file system.

I found this when I enabled nanostack with CMake but kvstore was not, this would cause a compilation error eventhough the conf for filesystem was set to 0.

Impact of changes

Migration actions required

Documentation

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[x] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@artokin

[0xc0170]

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