AK: Make Utf8View constexpr-constructible

Author: https://github.com/IdanHo
Commit: https://github.com/SerenityOS/serenity/commit/4774bed5894
Pull-request: https://github.com/SerenityOS/serenity/pull/11950
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/linusg

AK/Utf8View.h

@@ -65,7 +65,7 @@ public:
     {
     }
 
-    explicit Utf8View(StringView string)
+    explicit constexpr Utf8View(StringView string)
         : m_string(string)
     {
     }