Fix the med-format helper.

In C++11, members with static initializers cause problems for construction with an initializer list: Here is an explanation from 1:

Why Does It Not Compile Under C++11? To be able to use the C structure initialization via curly braces, the structure needs to be an aggregate. In terms of structures, this means that the structure cannot have any private/protected members, user-provided constructors, base classes, or virtual member functions (see this great post for an explanation). However, C++11 added a new requirement: for a structure to be an aggregate, there cannot be any initializers for non-static members [ISO C++11, 8.5.1, §1].