cgen: fix auto str for fn struct member (#21825)

vlib/v/gen/c/auto_str_methods.v

@@ -1142,8 +1142,7 @@ fn struct_auto_str_func(sym &ast.TypeSymbol, lang ast.Language, _field_type ast.
 		}
 		return 'indent_${fn_name}(${obj}, indent_count + 1)', true
 	} else if sym.kind == .function {
-		obj := '${deref}it${op}${final_field_name}${sufix}'
-		return '${fn_name}(${obj})', true
+		return '${fn_name}()', true
 	} else if sym.kind == .chan {
 		return '${fn_name}(${deref}it${op}${final_field_name}${sufix})', true
 	} else if sym.kind == .thread {

vlib/v/gen/c/testdata/struct_fn_member_print.c.must_have

@@ -0,0 +1,3 @@
+static string main__MyStruct_str(main__MyStruct it) { return indent_main__MyStruct_str(it, 0);}
+static string main__Function_str() { return _SLIT("fn (int)");}
+string _t1 = main__Function_str();

vlib/v/gen/c/testdata/struct_fn_member_print.out

@@ -0,0 +1,3 @@
+MyStruct{
+    func: fn (int)
+}

vlib/v/gen/c/testdata/struct_fn_member_print.vv

@@ -0,0 +1,18 @@
+module main
+
+type Function = fn (int)
+
+struct MyStruct {
+        func Function @[required]
+}
+
+fn implementation(size int) {
+        println('size is ${size}')
+}
+
+fn main() {
+        m := MyStruct{
+                func: implementation
+        }
+        println('${m}')
+}