Fix building on wasm32 target.

See #469.
diff --git a/src/export.rs b/src/export.rs
index 148839e..cc7a222 100644
--- a/src/export.rs
+++ b/src/export.rs
@@ -12,7 +12,10 @@
 
 pub use span::IntoSpans;
 
-#[cfg(feature = "proc-macro")]
+#[cfg(all(
+    not(all(target_arch = "wasm32", target_os = "unknown")),
+    feature = "proc-macro"
+))]
 pub use proc_macro::TokenStream;
 
 #[cfg(feature = "printing")]