lxc: include sys/stat.h

This fixes the build on Debian Wheezy which otherwise fails with:

  CC     libvirt_driver_lxc_impl_la-lxc_process.lo
  lxc/lxc_process.c: In function 'virLXCProcessGetNsInode':
  lxc/lxc_process.c:648:5: error: implicit declaration of function 'stat' [-Werror=implicit-function-declaration]
  lxc/lxc_process.c:648:5: error: nested extern declaration of 'stat' [-Werror=nested-externs]
  cc1: all warnings being treated as errors

src/lxc/lxc_process.c

@@ -21,6 +21,7 @@
 
 #include <config.h>
 
+#include <sys/stat.h>
 #include <unistd.h>
 #include <fcntl.h>
 #include <signal.h>