MdeModulePkg RegularExpressionDxe: Add the missing EFIAPI for the function

MdeModulePkg RegularExpressionDxe: Add the missing EFIAPI for the function

The function with the variable parameters should have EFIAPI.

Cc: Feng Tian <feng.tian@intel.com>
Cc: Cinnamon Shia <cinnamon.shia@hpe.com>
Cc: Cecil Sheng <cecil.sheng@hpe.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>