Prevent building spatialaudio_quick3d if quick3d is installed and the feature
is disabled.

--- src/CMakeLists.txt.orig	2025-08-15 19:16:13 UTC
+++ src/CMakeLists.txt
@@ -27,7 +27,7 @@ endif()
 if(TARGET Qt::Quick)
     add_subdirectory(multimediaquick)
 endif()
-if(TARGET Qt::Quick3D AND QT_FEATURE_spatialaudio)
+if(TARGET Qt::Quick3D AND QT_FEATURE_spatialaudio AND QT_FEATURE_spatialaudio_quick3d)
     add_subdirectory(spatialaudioquick3d)
 endif()