fix(nuttx): fix compile warning (#6156)

Signed-off-by: Xu Xingliang <xuxingliang@xiaomi.com>

src/drivers/nuttx/lv_nuttx_entry.h

@@ -47,6 +47,8 @@ typedef struct _lv_nuttx_ctx_t {
 
 #if LV_CACHE_DEF_SIZE > 0
     void * image_cache;
+#else
+    void * dummy;
 #endif
 
 } lv_nuttx_ctx_t;