OvmfPkg/OvmfXen: Fix Xen build

OvmfPkg/OvmfXen: Fix Xen build

commit 202fb22be6e9 "OvmfPkg/SecMain: validate the memory used for
decompressing Fv" broke building OvmfXen with:

edk2/OvmfPkg/OvmfXen.dsc(...): error 1001: Module type [SEC] is not
supported by library instancer
[edk2/OvmfPkg/Library/BaseMemEncryptSevLib/DxeMemEncryptSevLib.inf]
	consumed by [edk2/OvmfPkg/Sec/SecMain.inf]

The above commit added a reference to MemEncryptSevLib into SecMain.inf,
but OvmfXen.dsc doesn't have a MemEncryptSevLib entry for SEC.  Add one
like OvmfPkgX64.dsc has.

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: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien@xen.org>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Jason Andryuk <jandryuk@gmail.com>