fix(core, iOS): use namespaced iOS Pigeon header import

Author: SelaseKay

packages/firebase_core/firebase_core/ios/firebase_core/Sources/firebase_core/messages.g.m

@@ -4,7 +4,7 @@
 // Autogenerated from Pigeon (v26.3.4), do not edit directly.
 // See also: https://pub.dev/packages/pigeon
 
-#import "messages.g.h"
+#import "include/firebase_core/messages.g.h"
 
 #if TARGET_OS_OSX
 @import FlutterMacOS;

packages/firebase_core/firebase_core_platform_interface/pigeons/messages.dart

@@ -19,6 +19,9 @@ import 'package:pigeon/pigeon.dart';
         '../firebase_core/ios/firebase_core/Sources/firebase_core/include/firebase_core/messages.g.h',
     objcSourceOut:
         '../firebase_core/ios/firebase_core/Sources/firebase_core/messages.g.m',
+    objcOptions: ObjcOptions(
+      headerIncludePath: 'include/firebase_core/messages.g.h',
+    ),
     cppHeaderOut: '../firebase_core/windows/messages.g.h',
     cppSourceOut: '../firebase_core/windows/messages.g.cpp',
     cppOptions: CppOptions(namespace: 'firebase_core_windows'),