uv_type: supress -Werror=unused compiler errors

The `uv_type()` constructor does not use its `token` parameter,
which causes a `-Werror=unused` compiler error on GCC (and probably
on other compilers too).

Luckily, C++17 adds the [`[[maybe_unused]]`][1] attribute that all
standards compliant compilers will support to hide unused errors.

[1]: https://en.cppreference.com/w/cpp/language/attributes/maybe_unused

on-behalf-of: @nqminds <[email protected]>

src/uvw/uv_type.hpp

@@ -16,7 +16,7 @@ namespace uvw {
  */
 template<typename U>
 struct uv_type {
-    explicit uv_type(loop::token token, std::shared_ptr<loop> ref) noexcept
+    explicit uv_type([[maybe_unused]] loop::token token, std::shared_ptr<loop> ref) noexcept
         : owner{std::move(ref)}, resource{} {}
 
     uv_type(const uv_type &) = delete;