LibGfx: Mark Vector3 class final

Author: https://github.com/sunverwerth
Commit: b0e5e78a01
Pull-request: https://github.com/SerenityOS/serenity/pull/6375
Reviewed-by: https://github.com/Hendiadyoin1
Reviewed-by: https://github.com/IdanHo
Reviewed-by: https://github.com/Quaker762
Reviewed-by: https://github.com/alimpfard
Reviewed-by: https://github.com/ccapitalK
Reviewed-by: https://github.com/gmta

Userland/Libraries/LibGfx/Vector3.h

@@ -10,7 +10,7 @@
 
 namespace Gfx {
 template<typename T>
-class Vector3 {
+class Vector3 final {
 public:
     Vector3() = default;
     Vector3(T x, T y, T z)