[C++] [cpprest] Fixed wstring on linux

[etherealjoy]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

Fixed compilation on Linux as commented here
#3572 (comment)

Ran the petstore test as well.

Thanks to @TomerJLevy for pointing out the issue

@MartinDelille @muttleyxd @stkrwork @ravinikam