Remove incorrect const qualifier in serializer methodreturn

[ewertons]

Checklist

• I have read the [contribution guidelines] (https://github.com/Azure/azure-iot-sdk-c/blob/master/.github/CONTRIBUTING.md).
• I added or modified the existing tests to cover the change (we do not allow our test coverage to go down).
• If this is a modification that impacts the behavior of a public API
  • I edited the corresponding document in the devdoc folder and added or modified requirements.
• I submitted this PR against the correct branch:
  • This pull-request is submitted against the master branch.
  • I have merged the latest master branch prior to submission and re-merged as needed after I took any feedback.
  • I have squashed my changes into one with a clear description of the change.

Reference/Link to the issue solved with this PR (if any)

Description of the problem

Compilation fails on Mac OS with:

Testing with xcodebuild.
-> AzureIoTHubClient (1.3.6-public-preview-c)
- WARN | source: Git sources should specify a tag.
- ERROR | xcodebuild: Returned an unsuccessful exit code.
- ERROR | xcodebuild: AzureIoTHubClient/serializer/src/methodreturn.c:70:41: error: cannot assign to non-static data member 'statusCode' with const-qualified type 'const int'
- NOTE | xcodebuild: AzureIoTHubClient/inc/methodreturn.h:16:51: note: non-static data member 'statusCode' declared const here

Description of the solution

Remove the const qualifiers.