Fix non-gc build

Author: alexcrichton

crates/wasmtime/src/runtime/store.rs

@@ -1573,7 +1573,7 @@ impl StoreOpaque {
         }
 
         #[cfg(not(feature = "gc"))]
-        fn allocate_gc_store(_: &mut StoreOpaque) -> Result<GcStore> {
+        async fn allocate_gc_store(_: &mut StoreOpaque) -> Result<GcStore> {
             bail!("cannot allocate a GC store: the `gc` feature was disabled at compile time")
         }
     }