chore: Add the missing include header that defines TARGET_OS_OSX

[ChristianEdwardPadilla]

Description

Add the missing include header that defines TARGET_OS_OSX.

Related Issues

None.

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Breaking Change

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[russellwheatley]

@ChristianEdwardPadilla is this necessary since I believe the target conditionals are already pulled in from the firebase-ios-sdk?

[ChristianEdwardPadilla]

@russellwheatley We are seeing issues during cross-compilation. When switching to cross-compiling, the newer version of clang cross compiler introduces the warning flag "-Wundef-prefix=TARGET_OS_". So the import needs to be explicit.

[russellwheatley]

Hey @ChristianEdwardPadilla, do you mind installing melos and running melos run format in the root of the project, please? This ought to fix the CI failure. Then we can look to get this merged in.