Fix unused macro warning when compiling tests without trace feature.

src/lib.rs

@@ -56,7 +56,7 @@ extern crate libsqlite3_sys as ffi;
 extern crate lru_cache;
 #[macro_use]
 extern crate bitflags;
-#[cfg(test)]
+#[cfg(all(test, feature = "trace"))]
 #[macro_use]
 extern crate lazy_static;