Suppress dll-interface warnings (C4251) on windows

[iche033]

🦟 Bug fix

Fixes #1106

Summary

Suppress C4251 warnings on windows.

Example warning msgs:

'gz::rendering::v8::Ogre2DynamicRenderable::dataPtr': class 'std::unique_ptr>' needs to have dll-interface to be used by clients of class 'gz::rendering::v8::Ogre2DynamicRenderable'

The warnings are emitted when there are STL typed objects declared in header classes. The reason for this warning is explained here, specifically:

...emitted when you use a non-dllexported class X in a dllexported class Y

This is no longer an issue in gz-rendering9 windows builds as I believe that's because visibility changed to hidden by default. So suppressing the warnings in gz-rendering8 / harmonic.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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