MdePkg UefiLib: Make the event empty function public

MdePkg UefiLib: Make the event empty function public

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=298

Then this event empty function can be used to remove
the duplication in drivers and other libraries.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>