Future-proof build fix for clang > 16

Clang is intending to remove the `-Wno-enum-constexpr-conversion` flag
in the future, because it can invoke undefined behavior.

To avoid the "integer value NNNN is outside the valid range of values
[0, 7] for the enumeration type 'ObjectDescriptionType'" warnings,
explicitly specify that the `ObjectDescriptionType` enum uses `unsigned`
as the underlying type.

https://github.com/KDE/phonon/commit/069d5a3eb81bcbc730e44c27a0d060a85cf9213d
--- phonon/objectdescription.h
+++ phonon/objectdescription.h
@@ -43,7 +43,7 @@ namespace Phonon
      *
      * \ingroup Backend
      */
-    enum ObjectDescriptionType
+    enum ObjectDescriptionType : unsigned
     {
         /**
          * Audio output devices. This can be soundcards (with different drivers), soundservers or