Add some missing include files which break build in certain platforms

Add some missing include files which break build in certain platforms

Two files were using functions from <sys/stat.h> but not including
in. Most of the time they got this automatically via another header,
but certain build flag combinations can reveal the problem

* src/lxc/lxc_container.c, src/node_device/node_device_linux_sysfs.c:
  Add <sys/stat.h>