MdeModulePkg RegularExpressionDxe: Enable STDARG for variable arguments

Set macro for variable arguments, and remove the hard code definition
to avoid the potential duplicated definition.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Dongao Guo <dongao.guo@intel.com>

MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regerror.c

@@ -28,7 +28,6 @@
  */
 
 #include "regint.h"
-#define HAVE_STDARG_PROTOTYPES
 #if 0
 #include <stdio.h> /* for vsnprintf() */