cgen: fix for/in codegen when iterating over C structs (#21052)

vlib/v/gen/c/for.v

@@ -435,7 +435,7 @@ fn (mut g Gen) for_in_stmt(node_ ast.ForInStmt) {
 		}
 		t_var := g.new_tmp_var()
 		receiver_typ := g.unwrap_generic(next_fn.params[0].typ)
-		receiver_styp := g.typ(receiver_typ)
+		receiver_styp := g.cc_type(receiver_typ, false)
 		mut fn_name := receiver_styp.replace_each(['*', '', '.', '__']) + '_next'
 		receiver_sym := g.table.sym(receiver_typ)
 		if receiver_sym.info is ast.Struct {

vlib/v/tests/c_structs/cstruct_iterator_test.c.v

@@ -0,0 +1,22 @@
+#include "@VMODROOT/iterator.h"
+
+struct C.MyCStruct {
+mut:
+	x int
+}
+
+fn (mut self C.MyCStruct) next() ?int {
+	if self.x >= 10 {
+		return none
+	}
+	self.x++
+	return self.x
+}
+
+fn test_iterating_over_cstructs() {
+	iter := C.MyCStruct{}
+	for x in iter {
+		println(x)
+		assert true
+	}
+}

vlib/v/tests/c_structs/iterator.h

@@ -0,0 +1,3 @@
+struct MyCStruct {
+        int x;
+};