add test compile_error

Author: newpavlov

kuznyechik/src/lib.rs

@@ -54,3 +54,6 @@ type KeySize = U32;
 pub type Block = GenericArray<u8, U16>;
 /// 256-bit Kuznyechik key
 pub type Key = GenericArray<u8, U32>;
+
+#[cfg(kuznyechik_force_soft)]
+compile_error!("kuznyechik_force_soft is enabled");