Fix build with libc++19; backported from openmw-0.49.0

Index: components/to_utf8/to_utf8.hpp
--- components/to_utf8/to_utf8.hpp.orig
+++ components/to_utf8/to_utf8.hpp
@@ -3,6 +3,7 @@
 
 #include <string>
 #include <cstring>
+#include <span>
 #include <vector>
 #include <string_view>
 
@@ -46,7 +47,7 @@ namespace ToUTF8
             inline std::pair<std::size_t, bool> getLengthLegacyEnc(std::string_view input) const;
             inline void copyFromArrayLegacyEnc(std::string_view::iterator& chp, std::string_view::iterator end, char* &out) const;
 
-            const std::basic_string_view<signed char> mTranslationArray;
+            const std::span<const signed char> mTranslationArray;
     };
 
     class Utf8Encoder
