OvmfPkg: LockBoxLib: -D SMM_REQUIRE excludes our fake lockbox

OvmfPkg: LockBoxLib: -D SMM_REQUIRE excludes our fake lockbox

When the user builds OVMF with -D SMM_REQUIRE, our LockBox implementation
must not be used, since it doesn't actually protect data in the LockBox
from the runtime guest OS. Add an according assert to
LockBoxLibInitialize().

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19046 6f19259b-4bc3-4df7-8a09-765794883524