Note that in the `sway_keyboard_configure` function of sway/input/keyboard.c,
we have skipped the `sway_keyboard_set_layout` function for virtual
keyboards, which then have null keymaps.
Hence, a null-safety check is needed at runtime.
The only reason it's included there is for a declaration of
struct sway_server, but we can just forward-declare it.
This avoids rebuilding almost all of Sway when touching server.h.
All other server.h includes are from source files, not headers.