Applied MarshalAs to wasmtime_engine_is_pulley to fix incorrect marshalling (#340)

Author: martindevans

src/Engine.cs

@@ -92,6 +92,7 @@ private static class Native
             public static extern void wasmtime_engine_increment_epoch(Handle engine);
 
             [DllImport(LibraryName)]
+            [return: MarshalAs(UnmanagedType.I1)]
             public static extern bool wasmtime_engine_is_pulley(Handle engine);
         }