kde: ECMQmlModule6 — make Qt6::Qml optional (qtdeclarative not on Redox)

find_package(Qt6 COMPONENTS Core Qml CONFIG) marked Qml as a required component,
so any KDE package including ECMQmlModule failed its feature_summary even though
the module already returns gracefully when Qt6::Qml is absent. Use
OPTIONAL_COMPONENTS for Qml (Core stays required). qtdeclarative/QtQuick/QtQml
are not built for the Redox cross target; QML modules are simply skipped.

local/recipes/kde/kf6-extra-cmake-modules/source/modules/ECMQmlModule6.cmake

@@ -7,10 +7,14 @@
 
 cmake_policy(VERSION 3.16)
 
-find_package(Qt6 COMPONENTS Core Qml CONFIG)
+# Qml is OPTIONAL: qtdeclarative is not built for the Redox cross target. Marking
+# it required here makes the including project's feature_summary fail even though
+# the block below already returns gracefully when Qt6::Qml is absent (QML modules
+# are simply not built; the C++ library still is).
+find_package(Qt6 COMPONENTS Core OPTIONAL_COMPONENTS Qml CONFIG)
 
 if (NOT TARGET Qt6::Qml)
-    message(WARNING "Target Qt6::Qml was not found. ECMQmlModule requires the QML module when building with Qt 6")
+    message(STATUS "Qt6::Qml not found; ECMQmlModule QML-module support disabled")
     return()
 endif()