Fix iOS CocoaPods wrapper double compile

[DavidPluxia]

Summary

• exclude the SwiftPM-only flutter_soloud.mm wrapper from the iOS CocoaPods target
• keep the CMake script phase as the single CocoaPods native implementation path
• avoid duplicate symbols when the app target force-loads libflutter_soloud_plugin.a

Details

flutter_soloud.mm includes the full C++ implementation so SwiftPM can build the package from that wrapper source. The iOS CocoaPods podspec also builds the implementation through build_cmake.sh and then force-loads libflutter_soloud_plugin.a into the app target. When CocoaPods also compiles flutter_soloud.mm, the same implementation is present twice.

This keeps the wrapper file in the package for SwiftPM, but uses CocoaPods' exclude_files to keep it out of the CocoaPods compile sources.

Validation

• ruby -c ios/flutter_soloud.podspec
• pod ipc spec ios/flutter_soloud.podspec
• NO_XIPH_LIBS=1 pod ipc spec ios/flutter_soloud.podspec
• local CocoaPods fixture with a stub Flutter pod: pod install, then verified generated Pods.xcodeproj references miniaudio_objc_prefix.h and libflutter_soloud_plugin.a, but no flutter_soloud.mm

[alnitak]

Hi @DavidPluxia, thanks a lot for this PR!

Do you think this fix should also be made for macOS?