Fix unused_mut warning when not compiling for wasm

examples/web.rs

@@ -8,6 +8,7 @@ use winit::{
 pub fn main() {
     let event_loop = EventLoop::new();
 
+    #[cfg_attr(not(target_arch = "wasm32"), allow(unused_mut))]
     let mut builder = WindowBuilder::new()
         .with_title("A fantastic window!")
         // We don't know if `css-size` is enabled or not, so just use a fixed size.