SignedCapsulePkg/SignedCapsulePkg.dsc: Add RngLib mapping

SignedCapsulePkg/SignedCapsulePkg.dsc: Add RngLib mapping

Fix build breaks for all architectures by adding RngLib mapping.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Abner Chang <abner.chang@hpe.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Daniel Schaefer <daniel.schaefer@hpe.com>
Acked-by: Abner Chang <abner.chang@hpe.com>