get rid of unused mut warning on extension_names for non-apple targets

lib/src/renderer/vulkan/vulkan.rs

@@ -43,6 +43,7 @@ impl RendererBackendManager {
 
         let name = std::ffi::CString::new("gameboy").unwrap();
 
+        #[allow(unused_mut)]
         let mut extension_names = enumerate_required_extensions(display_handle)
             .unwrap()
             .to_vec();