COMP: Add missing qMRMLWidget header file

[sadhana-r]

This commit resolves the following error encountered during the custom app build process, by addressing changes introduced in Slicer/Slicer@fb6fb19. This change ensures the custom app build system works with the latest Slicer versions.

>C:\W\MPs\Applications\CustomApp\qCustomAppMainWindow.cxx(83,24): error C2653: 'qMRMLWidget': is not a class or namespace name [C:\W\MPR_Test\Slicer-build\Applications\CustomApp\qCustomApp.vcxproj]
>C:\W\MPs\Applications\CustomApp\qCustomAppMainWindow.cxx(83,37): error C3861: 'pixmapFromIcon': identifier not found [C:\W\MPR_Test\Slicer-build\Applications\CustomApp\qCustomApp.vcxproj]