MdePkg/UefiDebugLibStdErr: make global variable static

MdePkg/UefiDebugLibStdErr: make global variable static

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1740

Add a 'static' storage-class specifier to the global variables
that only used in a single file to minimize the name collisions.
This is only for the variable named 'mExitBootServicesEvent'.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>