fix weird undeclared bug?

src/netif.rs

@@ -599,16 +599,18 @@ impl EspNetif {
         Ok(())
     }
 
-    #[cfg(esp_idf_lwip_ipv4_napt)]
     unsafe extern "C" fn napt_wrapper(ctx: *mut ffi::c_void) {
-        let ctx = unsafe { *(ctx as *mut (u8, i32)) };
+        #[cfg(esp_idf_lwip_ipv4_napt)]
+        {
+            let ctx = unsafe { *(ctx as *mut (u8, i32)) };
 
-        ip_napt_enable_no(ctx.0, ctx.1);
+            ip_napt_enable_no(ctx.0, ctx.1);
+        }
     }
 
     /// Enables or disables NAPT on this netif.
     ///
-    /// Enable operation can be performed only on one interface at a time. 
+    /// Enable operation can be performed only on one interface at a time.
     /// NAPT cannot be enabled on multiple interfaces according to this implementation.
     #[cfg(esp_idf_lwip_ipv4_napt)]
     pub fn enable_napt(&mut self, enable: bool) -> Result<(), EspError> {