The SMM based FTW and variable drivers are going to depend on
MmServicesTableLib after a subsequent patch, so add a resolution
for it to MdeModulePkg.dsc.
Contributed-under: TianoCore Contribution Agreement 1.1 Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Reviewed-by: Jian J Wang <jian.j.wang@intel.com> Reviewed-by: Star Zeng <star.zeng@intel.com>