fix: suppress warning for link_args

this is a bug at https://github.com/rust-lang/rust/issues/55895
diff --git a/samplecode/switchless/app/src/main.rs b/samplecode/switchless/app/src/main.rs
index 4de1afd..8876218 100644
--- a/samplecode/switchless/app/src/main.rs
+++ b/samplecode/switchless/app/src/main.rs
@@ -16,6 +16,7 @@
 // under the License..
 
 #![feature(link_args)]
+#![allow(unused_attributes)]
 #![link_args = "-Wl,--whole-archive -lsgx_uswitchless -Wl,--no-whole-archive"]
 
 extern crate sgx_types;