Add no-op session

This is useful for headless testing, for instance with VKMS:

    modprobe vkms
    export WLR_DRM_DEVICES=/dev/dri/card1
    export WLR_BACKENDS=drm
    export LIBSEAT_BACKEND=noop
    sway

We don't need any of the VT handling in this case.

include/libseat.h

@@ -1,6 +1,8 @@
 #ifndef _LIBSEAT_H
 #define _LIBSEAT_H
 
+#include <stdarg.h>
+
 /*
  * An opaque struct containing an opened seat, created by libseat_open_seat and
  * destroyed by libseat_close_seat.