Fix the warning:

start.c: In function ‘lxc_fini’:
start.c:250: warning: implicit declaration of function ‘lxc_unlink_nsgroup’
start.c: In function ‘lxc_spawn’:
start.c:380: warning: implicit declaration of function ‘lxc_rename_nsgroup’
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>