MinPlatformPkg: Add missing macro for LargeVariableLib

With all the refactoring done during code review, I misplaced
the MAX_VARIABLE_NAME_PAD_SIZE macro. This patch adds it back.

Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Michael Kubacki <michael.kubacki@microsoft.com>
Cc: Isaac Oram <isaac.w.oram@intel.com>
Signed-off-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>