C++ SDK no destructor body

[nikibobi]

For some time now I got weird linker errors when I try to delete pointer to agones::SDK. When I looked at the source I saw that there is a destructor declared in the .h file, but it is missing in the .cpp file.

There are a few ways to fix it:

1. Delete the declaration in the header file: ~SDK();
2. Add empty body in cpp file: agones::SDK::~SDK() { }
3. Set it equal to default in the header file: ~SDK() = default;

[markmandel]

Please submit a PR! My c++ is not good, as you can see!