[PSDK] Remove default value for BSTR parameter in msxml2.idl

Fixes GCC 8 warning:
In file included from base/applications/msconfig_new/xmldomparser.hpp:26,
                 from base/applications/msconfig_new/toolspage.cpp:11:
sdk/include/psdk/msxml2.h:12705:20: error: ISO C++ forbids converting a string constant to 'BSTR' {aka 'wchar_t*'} [-Werror=write-strings]
         BSTR uri = L"x") = 0;
                    ^~~~

sdk/include/psdk/msxml2.idl

@@ -1258,7 +1258,7 @@ interface IXSLProcessor : IDispatch
     [id(DISPID_XMLDOM_PROCESSOR_SETSTARTMODE)]
     HRESULT setStartMode(
         [in] BSTR p, 
-        [in, defaultvalue(L"")] BSTR uri);
+        [in] BSTR uri);
 
     [propget, id(DISPID_XMLDOM_PROCESSOR_STARTMODE)]
     HRESULT startMode([retval, out] BSTR *p);
@@ -1286,7 +1286,11 @@ interface IXSLProcessor : IDispatch
     HRESULT addParameter(
         [in] BSTR p, 
         [in] VARIANT var, 
+#ifndef __REACTOS__
         [in, defaultvalue(L"")] BSTR uri);
+#else
+        [in] BSTR uri);
+#endif
 
     [id(DISPID_XMLDOM_PROCESSOR_ADDOBJECT)]
     HRESULT addObject(