Add missing Extern "C" to MSVC portmacro.h

Add missing Extern "C" to MSVC portmacro.h as a fix for issue #1064

Description

It appears that in merge request https://github.com/FreeRTOS/FreeRTOS-Kernel/pull/1047 The C++ block was reduced to not include the transitive header includes. This makes sense but revealed the lack of the extern "C" guard in the MSVC-MingW/portmacro.h header.

Test Steps

compile a project for the MSVC-MingW simulator and there will be undefined references.

Checklist:

[ ] I have tested my changes. No regression in existing tests.
[ ] I have modified and/or added unit-tests to cover the code changes in this Pull Request.

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FreeRTOS / FreeRTOS-Kernel