CMake: Write Qt's namespace into module info JSON files

If Qt was configure with QT_NAMESPACE set, store that in the module
information files under 'built_with.namespace'.

Task-number: QTBUG-111158
Change-Id: I273309cb263c64f801dbb7238440336d7afa635e
Reviewed-by: Amir Masoud Abdol <amir.abdol@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtModuleHelpers.cmake

@@ -1093,6 +1093,10 @@ function(qt_describe_module target)
     endif()
 
     set(extra_build_information "")
+    if(NOT QT_NAMESPACE STREQUAL "")
+        string(APPEND extra_build_information "
+        \"namespace\": \"${QT_NAMESPACE}\",")
+    endif()
     if(ANDROID)
         string(APPEND extra_build_information "
         \"android\": {