CPack: Fix dashboard errors (#11575)

Use same technique here with UuidToString as already found in
cmGlobalVisualStudio7Generator::CreateGUID to avoid using a type
that is not defined on older systems... (VS 7.0 and Borland)

Source/CPack/WiX/cmCPackWIXGenerator.cxx

@@ -539,8 +539,8 @@ std::string cmCPackWIXGenerator::GenerateGUID()
   UUID guid;
   UuidCreate(&guid);
 
-  RPC_CSTR tmp = 0;
-  UuidToStringA(&guid, &tmp);
+  unsigned char *tmp = 0;
+  UuidToString(&guid, &tmp);
 
   std::string result(reinterpret_cast<char*>(tmp));
   RpcStringFree(&tmp);