CMake: Install plugins on Windows

The plugin dll files were missing from install_dir/plugins/

Change-Id: I68655faf949e4b8fcab153c6c9b8ee14d3ad8ecc
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Qt CMake Build Bot

cmake/QtBuild.cmake

@@ -1477,6 +1477,7 @@ function(add_qt_plugin target)
     set(export_name "${target}Targets")
     qt_install(TARGETS "${target}"
                EXPORT ${export_name}
+               RUNTIME DESTINATION "${install_directory}"
                LIBRARY DESTINATION "${install_directory}"
                ARCHIVE DESTINATION "${archive_install_directory}"
     )