fix: fix compile error related to MxcImage

We ran into this error after updating Flutter from 3.27 to 3.32:
```
lib/widgets/mxc_image.dart:119:16: Error: The getter '_' isn't defined for the class '_MxcImageState'.
     - '_MxcImageState' is from 'package:fluffychat/widgets/mxc_image.dart' ('lib/widgets/mxc_image.dart').
    Try correcting the name to the name of an existing getter, or defining a getter or field named '_'.
          _tryLoad(_);
```

This patch fixes that error.

lib/widgets/mxc_image.dart

@@ -107,7 +107,7 @@ class _MxcImageState extends State<MxcImage> {
     }
   }
 
-  void _tryLoad(_) async {
+  void _tryLoad() async {
     if (_imageData != null) {
       return;
     }
@@ -116,14 +116,14 @@ class _MxcImageState extends State<MxcImage> {
     } on IOException catch (_) {
       if (!mounted) return;
       await Future.delayed(widget.retryDuration);
-      _tryLoad(_);
+      _tryLoad();
     }
   }
 
   @override
   void initState() {
     super.initState();
-    WidgetsBinding.instance.addPostFrameCallback(_tryLoad);
+    WidgetsBinding.instance.addPostFrameCallback((_) => _tryLoad());
   }
 
   Widget placeholder(BuildContext context) =>