Fix incorrect stringstream syntax usage

The str() function is not being used to pull data out of stringstream mgs.  This causes the build to fail when trying to apply mgs to the << operand.

changelog

@@ -436,6 +436,7 @@ Version 1.13.0-dev:
    * Multiplayer server can now handle scenarios with more than 9 sides
    * [modify_side] controller= now works in networked mp. If a null-controlled side becomes alive by
      [modify_side] controller=ai/human then the currently active client will be assiged controll
+   * Fixed a stingstream syntax error that caused the build to fail in Visual Studio 13
 
 Version 1.11.11:
  * Add-ons server:

data/core/about.cfg

@@ -904,6 +904,9 @@
     [entry]
         name = "Aaron Keisch-Walter (Exasperation)"
     [/entry]
+    [entry]
+        name = "Adam Leffew"
+    [/entry]
     [entry]
         name = "Adrian Iosif (Zappaman)"
     [/entry]

src/game_initialization/configure_engine.cpp

@@ -20,7 +20,7 @@ configure_engine::configure_engine(saved_game& state) :
 	if (sides_.first == sides_.second) {
 		std::stringstream msg;
 		msg << "Configure Engine: No sides found in scenario, aborting.";
-		std::cerr << msg;
+		std::cerr << msg.str();
 		std::cerr << "Full scenario config:\n";
 		std::cerr << state_.to_config().debug();
 		throw game::error(msg.str());