MdePkg Acpi51.h:Correct the declaration of EFI_ACPI_5_1_GTDT_SBSA_GENERIC_WATCHDOG_STRUCTURE.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed by: Jiewen Yao <jiewen.yao@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdePkg@15961 6f19259b-4bc3-4df7-8a09-765794883524