OvmfPkg: Add dependency of VariableSmm driver to make it work normally.

OvmfPkg: Add dependency of VariableSmm driver to make it work normally.

REF? https://bugzilla.tianocore.org/show_bug.cgi?id=3861

UefiCpuPkg define a new Protocol with the new services
SmmWaitForAllProcessor(), which can be used by SMI handler
to optionally wait for other APs to complete SMM rendezvous in
relaxed AP mode.

VariableSmm driver need use SmmCpuRendezvousLib, So add
SmmCpuRendezvousLib in OvmfPkg.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Sebastien Boeuf <sebastien.boeuf@intel.com>
Signed-off-by: Zhihao Li <zhihao.li@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>