MdePkg: Missing GUID variable in DiskInfo.h

MdePkg: Missing GUID variable in DiskInfo.h

The DiskInfo.h defines EFI_DISK_INFO_NVME_INTERFACE_GUID, but does not declare a corresponding EFI_GUID variable.
The attached patch adds "extern" statement for the variable in DiskInfo.h. It also adds variable definition to MdePkg.dec.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Felix Polyudov <felixp@ami.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17102 6f19259b-4bc3-4df7-8a09-765794883524