Use impl namespace in GattServer att_cb registration, not ble

[ghost]

Summary of changes

Fix compilation of apps when BLE is enabled.

If this is the correct fix, then this shouldn't have passed CI, should
it?

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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

[] 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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[ciarmcom]

@jrobeson, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[pan-]

Thanks @jrobeson The fix is correct. It highlight we should but more BLE configurations in the CI. Compiling without GattClient is fairly common.

[0xc0170]

CI started

[mbed-ci]

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