Compile Initialization with the game

[bitsandfoxes]

In preparation for the coming native support.
The aim is to have the Initialization compile with the game to have access to preprocessor directives.

[bruno-garcia]

Sweet! Surprisingly simple change.
One nit but other than that, LGTM

[github-actions]

	Fails
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- Compile Initialization with the game ([#272](https://github.com/getsentry/sentry-unity/pull/272))

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