Fix binary compatibility

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/message/GroupMessageEvent.kt

@@ -39,7 +39,7 @@ class GroupMessageEvent(
         check(source is OnlineMessageSource.Incoming.FromGroup) { "source provided to a GroupMessage must be an instance of OnlineMessageSource.Incoming.FromGroup" }
     }
 
-    inline val group: Group get() = sender.group
+    override val group: Group get() = sender.group
     override val bot: Bot get() = sender.bot
 
     override val subject: Group get() = group

mirai-core/src/commonMain/kotlin/net.mamoe.mirai/message/MessageEvent.kt

@@ -208,7 +208,9 @@ abstract class FriendMessage : MessageEvent()
     level = DeprecationLevel.ERROR
 )
 @Suppress("DEPRECATION_ERROR")
-abstract class GroupMessage : MessageEvent()
+abstract class GroupMessage : MessageEvent() {
+    abstract val group: Group
+}
 
 @PlannedRemoval("1.2.0")
 @Deprecated(