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/RegularExpressionDxe.inf

@@ -79,6 +79,9 @@
   gEfiRegularExpressionProtocolGuid       ## PRODUCES
 
 [BuildOptions]
+  # Enable STDARG for variable arguments
+  *_*_*_CC_FLAGS = -DHAVE_STDARG_H
+
   # Override MSFT build option to remove /Oi and /GL
   MSFT:*_*_*_CC_FLAGS          = /GL-
   INTEL:*_*_*_CC_FLAGS         = /Oi-