lantern: Fix incompatible redefinition of LANTERN_HOST_HANDLER

Fixes the following warning:

```
  In file included from device.cpp(2):
  torch_types.h(7): warning # 47: incompatible redefinition of macro "LANTERN_HOST_HANDLER" (declared at line 35 of "lantern/lantern.h")
    #define LANTERN_HOST_HANDLER lantern_host_handler();
```

Signed-off-by: Hamada S. Badr <hamada.s.badr@gmail.com>