vo_gpu/hwdec_cuda: fixup compilation with vulkan disabled

The actual code utilizing this enum was seemingly properly if'd,
but not the enum in the struct itself.

Fixes compilation.

video/out/opengl/hwdec_cuda.c

@@ -59,7 +59,9 @@ struct priv_owner {
                      const struct ra_format *format, int n);
     void (*ext_uninit)(struct ra_hwdec_mapper *mapper, int n);
 
+#if HAVE_VULKAN
     enum pl_handle_type handle_type;
+#endif
 };
 
 struct ext_gl {