Make dtMeshTile POD

[akortunov]

Fixes this warning:

/home/travis/build/OpenMW/openmw/extern/recastnavigation/Detour/Source/DetourNavMesh.cpp: In member function ‘dtStatus dtNavMesh::init(const dtNavMeshParams*)’:
/home/travis/build/OpenMW/openmw/extern/recastnavigation/Detour/Source/DetourNavMesh.cpp:243:50: warning: ‘void* memset(void*, int, size_t)’ clearing an object of type ‘struct dtMeshTile’ with no trivial copy-assignment [-Wclass-memaccess]
  memset(m_tiles, 0, sizeof(dtMeshTile)*m_maxTiles);
                                                  ^
In file included from /home/travis/build/OpenMW/openmw/extern/recastnavigation/Detour/Source/DetourNavMesh.cpp:22:
/home/travis/build/OpenMW/openmw/extern/recastnavigation/Detour/Include/DetourNavMesh.h:276:8: note: ‘struct dtMeshTile’ declared here
 struct dtMeshTile
        ^~~~~~~~~~

Private constructors without implementation (a pre-C++11 hack to mark constructors as "deleted") make dtMeshTile struct non-POD, so memset can not be used to initialize it.

The simplest solution is to remove such constructors, so dtMeshTile becomes POD, and compilers will not try to optimize data alignment.

R&D developers are not going to fix this issue in their upstream code.

[psi29a]

I'm not a fan of diverging from upstream on extern projects we pull in. Could you at least mark the code that has diverged, that way when we pull in upstream later, we'll see in the diff what code OpenMW has changed.

[akortunov]

#2645