Fix add_qt_gui_executable usage in offlineshaders example

Change-Id: Ie84d38415692b56a746f45f9a3e96eea30d52c15
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

examples/quick3d/offlineshaders/CMakeLists.txt

@@ -20,7 +20,7 @@ find_package(Qt6 COMPONENTS Gui)
 find_package(Qt6 COMPONENTS Quick)
 find_package(Qt6 COMPONENTS Quick3D)
 
-add_qt_gui_executable(offlineshaders
+qt_add_executable(offlineshaders
     main.cpp
 )
 target_link_libraries(offlineshaders PUBLIC