Hide the destructor of resource::Model
If "~Model() = default" is in the header file, the compilation fails
with the following message:
/usr/include/simgrid/kernel/resource/Model.hpp:35:11: required from here
/usr/include/c++/6/bits/unique_ptr.h:74:22: error: invalid application of 'sizeof' to incomplete type 'simgrid::kernel::lmm::System'
static_assert(sizeof(_Tp)>0,
That's too bad because simgrid::kernel::lmm::System is not a public
type. This fails in particular when compiling simgrid-FMI.
Hiding the destructor in the cpp as done with this commit works better
because lmm::System is visible from Model.cpp
