kde: breeze — build Qt6 style only + declare kf6-kcmutils

- The upstream CMakeLists defaults BUILD_QT5=ON and calls build_Qt5() (needs Qt5,
  which Redbear does not ship). Pass -DBUILD_QT5=OFF -DBUILD_QT6=ON.
- The window-decoration KCM uses kcmutils_generate_desktop_file(), a cmake macro
  from kf6-kcmutils — declare that dependency.

local/recipes/kde/breeze/recipe.toml

@@ -26,6 +26,8 @@ dependencies = [
     "kf6-kiconthemes",
     "kf6-kwidgetsaddons",
     "kf6-kwindowsystem",
+    # kcmutils_generate_desktop_file() cmake macro (window-decoration KCM).
+    "kf6-kcmutils",
     "kdecoration",
 ]
 script = """
@@ -54,6 +56,8 @@ cmake "${COOKBOOK_SOURCE}" \
     -DCMAKE_BUILD_TYPE=Release \
     -DCMAKE_PREFIX_PATH="${COOKBOOK_SYSROOT}" \
     -DBUILD_TESTING=OFF \
+    -DBUILD_QT5=OFF \
+    -DBUILD_QT6=ON \
     -DBUILD_QCH=OFF \
     -DBUILD_WITH_QML=OFF \
     -DUSE_DBUS=ON \