Tests: add missing mbed_trace.h multihoming header

[michalpasztamobica]

Summary of changes

The printf calls were righteously changed to mbed_* in #12267, but the multihoming tests were missing an include mbed_trace. This might have not been noticed, because multihoming tests have specific setup, which is not available in nightly tests and most likely they never really get built. Fortunately, we have an internal CI running, which detected this omission.

Impact of changes

Multihoming tests compile without errors again.

Migration actions required

None

Documentation

None

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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)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@AnttiKauppila
@AriParkkila
@tymoteuszblochmobica

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[ciarmcom]

@michalpasztamobica, thank you for your changes.
@AnttiKauppila @AriParkkila @tymoteuszblochmobica @ARMmbed/mbed-os-test @ARMmbed/mbed-os-ipcore @ARMmbed/mbed-os-maintainers please review.

[kjbracey]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 5 of 5 test jobs passed
Build number : 1
Build artifacts

[mergify]

This PR does not contain release version label after merging.

[0xc0170]

I've fixed the version: Set to 6.0.0-alpha-2