Exclude plugin sources that use sentry-native if build platform isn't supported

[tustanivsky]

Related to #32 and getsentry/sentry-docs#13861.

#skip-changelog

[tustanivsky]

@nwhite-riot To prevent build errors when packaging for unsupported platforms (e.g., Switch) or when the corresponding extension isn’t installed I’ve also added the SENTRY_COMPILED_PLATFORM_HEADER macro (see our previous discussion here).

At some point it might make sense to add platform-specific includes for all Sentry entities so we can use it consistently in HAL/PlatformSentryXXX.h. but for now having this in place for Subsystem and User should be sufficient.