performance_test: fix bad last argument calling add_arg

Coverity 182572

tests/performance_tests/main.cpp

@@ -75,10 +75,10 @@ int main(int argc, char** argv)
   const command_line::arg_descriptor<bool> arg_verbose = { "verbose", "Verbose output", false };
   const command_line::arg_descriptor<bool> arg_stats = { "stats", "Including statistics (min/median)", false };
   const command_line::arg_descriptor<unsigned> arg_loop_multiplier = { "loop-multiplier", "Run for that many times more loops", 1 };
-  command_line::add_arg(desc_options, arg_filter, "");
-  command_line::add_arg(desc_options, arg_verbose, "");
-  command_line::add_arg(desc_options, arg_stats, "");
-  command_line::add_arg(desc_options, arg_loop_multiplier, "");
+  command_line::add_arg(desc_options, arg_filter);
+  command_line::add_arg(desc_options, arg_verbose);
+  command_line::add_arg(desc_options, arg_stats);
+  command_line::add_arg(desc_options, arg_loop_multiplier);
 
   po::variables_map vm;
   bool r = command_line::handle_error_helper(desc_options, [&]()