SignedCapsulePkg: Consume MdeLibs.dsc.inc for RegisterFilterLib

SignedCapsulePkg: Consume MdeLibs.dsc.inc for RegisterFilterLib

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3246

MdeLibs.dsc.inc was added for some basic/default library
instances provided by MdePkg and RegisterFilterLibNull Library
was also added into it as the first version of MdeLibs.dsc.inc.

So update platform dsc to consume MdeLibs.dsc.inc for
RegisterFilterLibNull which will be consumed by IoLib and BaseLib.

Cc: Jian J Wang <jian.j.wang@intel.com>
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>