NetworkPkg/NetworkPkg.dsc: Add RngLib mapping for ARM and RISCV64

NetworkPkg/NetworkPkg.dsc: Add RngLib mapping for ARM and RISCV64

Fix NetworkPkg build breaks for ARM and RISCV64 by adding RngLib
mapping.

Cc: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Siyuan Fu <siyuan.fu@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>
Reviewed-by: Maciej Rabeda <maciej.rabeda@linux.intel.com>
Acked-by: Abner Chang <abner.chang@hpe.com>