On cross-build, use the host's binary.
Apply to both cross-build kdesignerplugin's package and use its package from others.
--- KF5DesignerPluginMacros.cmake.ORIG
+++ KF5DesignerPluginMacros.cmake
@@ -51,9 +51,14 @@
     get_filename_component(basename ${input} NAME_WE)
     set(source ${CMAKE_CURRENT_BINARY_DIR}/${basename}widgets.cpp)
 
+    set(KGENDESIGNERPLUGIN_LOCATION KF5::kgendesignerplugin)
+    if(CMAKE_CROSSCOMPILING)
+        set(KGENDESIGNERPLUGIN_LOCATION /usr/bin/kgendesignerplugin)
+    endif()
+
     # create source file from the .widgets file
     add_custom_command(OUTPUT ${source}
-        COMMAND KF5::kgendesignerplugin
+        COMMAND ${KGENDESIGNERPLUGIN_LOCATION}
         ARGS -o ${source} ${input}
         MAIN_DEPENDENCY ${input})
 
