util/IntrusiveForwardList: make end() non-static

So we can have a `const` overload which is important for some
algorithm templates.

src/util/IntrusiveForwardList.hxx

@@ -292,7 +292,7 @@ public:
 		return {head.next};
 	}
 
-	static constexpr iterator end() noexcept {
+	constexpr iterator end() noexcept {
 		return {nullptr};
 	}
 
@@ -357,6 +357,10 @@ public:
 		return {head.next};
 	}
 
+	constexpr const_iterator end() const noexcept {
+		return {nullptr};
+	}
+
 	constexpr const_iterator last() const noexcept
 		requires(options.cache_last) {
 		return {last_cache.value};