SpcrFeaturePkg: Close the event after first callback.

Adds CloseEvent in callback routine OutOfBandACPITableConstruction(),
to avoid multiple installation of SPCR table.

Cc: Isaac Oram <isaac.w.oram@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Abdul Lateef Attar <AbdulLateef.Attar@amd.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>