Add missing SWIFT_PACKAGE define for Apple platforms

[tustanivsky]

This PR resolves packaging errors in UE 5.4 and 5.5 due to a missing SWIFT_PACKAGE define when targeting Apple platforms. The problem was discovered after submitting version 1.0.0 to FAB.

[jpnurmi]

What's the error, out of curiosity? Shouldn't #if SWIFT_PACKAGE and #if !SWIFT_PACKAGE in sentry-cocoa headers work fine with a C++ compiler without explicitly defining SWIFT_PACKAGE=0? Does Unreal packaging process involve reading and parsing those headers with some custom tooling?

[tustanivsky]

What's the error, out of curiosity? Shouldn't #if SWIFT_PACKAGE and #if !SWIFT_PACKAGE in sentry-cocoa headers work fine with a C++ compiler without explicitly defining SWIFT_PACKAGE=0? Does Unreal packaging process involve reading and parsing those headers with some custom tooling?

error: 'SWIFT_PACKAGE' is not defined, evaluates to 0 [-Werror,-Wundef]

Unreal builds with -Wundef so missing defines trigger warnings which are promoted to errors via -Werror. Interestingly, UE 5.6 doesn’t fail on this which might indicate that Epic changed the default warning/error configuration for packaging in that version.