MdePkg/Include/Protocol/Tls.h: pack structures from the TLS RFC

MdePkg/Include/Protocol/Tls.h: pack structures from the TLS RFC

The structures defined in RFC 5246 are not to have any padding between
fields or at the end; use the "pack" pragma as necessary.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=915
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Long Qin <qin.long@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>