ensure clock_gettime symbol is found

got link error liblxc.so: undefined reference to `clock_gettime'
clock_gettime is used by lxclock.c and is in librt, or bionic libc.

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>