lxc/tools/lxc_monitor: include missing <stddef.h>

lxc/tools/lxc_monitor: include missing <stddef.h>

lxc_monitor.c uses offsetof(), so it should include
<stddef.h>. Otherwise the build fails with the musl C library:

tools/lxc_monitor.c: In function ‘lxc_abstract_unix_connect’:
tools/lxc_monitor.c:324:9: warning: implicit declaration of function ‘offsetof’ [-Wimplicit-function-declaration]
         offsetof(struct sockaddr_un, sun_path) + len + 1);
         ^~~~~~~~
tools/lxc_monitor.c:324:18: error: expected expression before ‘struct’
         offsetof(struct sockaddr_un, sun_path) + len + 1);
                  ^~~~~~

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>