AK: Make Deprecated{Fly,}String and StringImpl const-correct

Author: https://github.com/awesomekling
Commit: a56dfd5c77
Pull-request: https://github.com/SerenityOS/serenity/pull/17557
Reviewed-by: https://github.com/linusg

AK/DeprecatedFlyString.h

@@ -29,7 +29,7 @@ public:
     {
     }
 
-    static DeprecatedFlyString from_fly_impl(NonnullRefPtr<StringImpl> impl)
+    static DeprecatedFlyString from_fly_impl(NonnullRefPtr<StringImpl const> impl)
     {
         VERIFY(impl->is_fly());
         DeprecatedFlyString string;
@@ -91,7 +91,7 @@ public:
     }
 
 private:
-    RefPtr<StringImpl> m_impl;
+    RefPtr<StringImpl const> m_impl;
 };
 
 template<>