StandaloneMm: Link VarCheckPolicyLibStandaloneMm into VariableStandaloneMm

VariableSmmRuntimeDxe (NS side) sends variable policy commands to StMM
using gVarCheckPolicyLibMmiHandlerGuid. The MMI handler for this GUID
is registered by VarCheckPolicyLib's constructor. Without linking
VarCheckPolicyLibStandaloneMm into VariableStandaloneMm, no handler
exists for policy commands, causing MmiManage to return EFI_NOT_FOUND
which ASSERTs in DEBUG builds and silently fails in RELEASE.

OVMF's StandaloneMm path (OvmfPkgX64.dsc) includes this; the upstream
ARM reference DSCs (VExpress-FVP, Juno, PlatformStandaloneMmRpmb) have
not yet been updated.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: lizthegrey

Platform/SolidRun/StandAloneMm/StandaloneMm.dsc

@@ -202,6 +202,7 @@
       DevicePathLib|MdePkg/Library/UefiDevicePathLib/UefiDevicePathLibBase.inf
       VarCheckLib|MdeModulePkg/Library/VarCheckLib/VarCheckLib.inf
       NULL|MdeModulePkg/Library/VarCheckUefiLib/VarCheckUefiLib.inf
+      NULL|MdeModulePkg/Library/VarCheckPolicyLib/VarCheckPolicyLibStandaloneMm.inf
       NULL|Silicon/NXP/Drivers/EepromFvb/FixupPcd.inf
       VariablePolicyLib|MdeModulePkg/Library/VariablePolicyLib/VariablePolicyLib.inf
       VariablePolicyHelperLib|MdeModulePkg/Library/VariablePolicyHelperLib/VariablePolicyHelperLib.inf