Fix type of gc_obj field in union WASMValue (#2177)

Change `wasm_obj_t *gc_obj` to `wasm_obj_t gc_obj`

core/iwasm/include/gc_export.h

@@ -73,7 +73,7 @@ typedef union WASMValue {
     float f32;
     double f64;
     V128 v128;
-    wasm_obj_t *gc_obj;
+    wasm_obj_t gc_obj;
     uint32_t type_index;
     struct {
         uint32_t type_index;

core/iwasm/interpreter/wasm.h

@@ -177,7 +177,7 @@ typedef union WASMValue {
     float64 f64;
     V128 v128;
 #if WASM_ENABLE_GC != 0
-    wasm_obj_t *gc_obj;
+    wasm_obj_t gc_obj;
     uint32 type_index;
     struct {
         uint32 type_index;