ShellPkg: Add the missing VariablePolicyHelperLib in ShellPkg.dsc
This change is required by f4b7b473b4afd0093768905529bfae09a2061d41.
Signed-off-by: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>