[ios] Preserve bcsymbolmap files when using CocoaPods

Bitcode symbol map files (`bcsymbolmap`) are used in conjunction with dSYMs to symbolicate crashes. CocoaPods doesn't yet preserve these or add them to an app's `xcarchive`, so the developer needs to do this themselves (typically as an install-only build copy phase).

platform/ios/Mapbox-iOS-SDK.podspec

@@ -27,4 +27,6 @@ Pod::Spec.new do |m|
   m.vendored_frameworks = 'dynamic/Mapbox.framework'
   m.module_name = 'Mapbox'
 
+  m.preserve_path = '**/*.bcsymbolmap'
+
 end