Platform/ARM: VExpressPkg fixes to work with new VariablePolicyLib

Update .dsc.inc by adding mappings for:
- SafeIntLib
- VariablePolicyLib
- VariablePolicyHelperLib
- VariablePolicyLib (runtime)

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Thomas Abraham <thomas.abraham@arm.com>
Signed-off-by: Leif Lindholm <leif@nuviainc.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@arm.com>