SecurityPkg Tcg2Dxe: Add Variable Arch protocol dependency

SecurityPkg Tcg2Dxe: Add Variable Arch protocol dependency

commit a7e2d20193e853020a1415c25b53280955055394 introduces the code to
get PcdTpm2AcpiTableRev in the driver entry point. This PCD is designed as
DynamicHii or DynamicHiiEx PCD. So, this PCD depends on Variable service.
To make sure PcdTpm2AcpiTableRev value be got, add Variable service as Depex.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: jiewen Yao <jiewen.yao@intel.com>