util/Cast: add "const" overload

src/util/Cast.hxx

@@ -93,4 +93,17 @@ ContainerCast(A &a, A C::*member)
 	return *OffsetCast<C, A>(&a, ContainerAttributeOffset<C, A>(member));
 }
 
+/**
+ * Cast the given pointer to a struct member to its parent structure.
+ */
+template<class C, class A>
+#if defined(__clang__) || GCC_CHECK_VERSION(4,7)
+constexpr
+#endif
+static inline const C &
+ContainerCast(const A &a, A C::*member)
+{
+	return *OffsetCast<const C, const A>(&a, ContainerAttributeOffset<C, A>(member));
+}
+
 #endif