Fixed compilation on IBM XLC due to incorrect function call

Templated functions dependent of a templated class must be prefixed with
the "template" keyword.

Foundation/include/Poco/Dynamic/Struct.h

@@ -190,7 +190,7 @@ class Struct
 	std::string toString()
 	{
 		std::string str;
-		Var(*this).convert<std::string>(str);
+		Var(*this).template convert<std::string>(str);
 		return str;
 	}