TypedObjectPlug : Don't use "extern" on Windows
TypedObjectPlug : Don't use "extern" on Windows

- Marking variables as "extern" in MSVC generates this warning: https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4910?view=msvc-160
- The "extern"s were introduced to fix a Mac error, so it is safe, and desireable, to exclude these on Windows (https://github.com/ImageEngine/cortex/commit/d6d9e783a36f07dfa1d0e67f929ec75f45cb4ab0#diff-bb99ebf651102989cbabc86eb2c276d7f986964d779a679c3541f8be14e21615)

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