Platform/LeMaker: Fix missing dependency on VariablePolicyHelperLib

The CelloBoard platform firmware build breaks due to the missing
dependency on VariablePolicyHelperLib.

Therefore, to satisfy the dependency include VariablePolicyHelperLib
in the LibraryClasses.common section.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Pierre Gondois <pierre.gondois@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>