api: fix build warning

api: fix build warning

Fix a build warning:
cgcreate.c: In function ‘create_systemd_scope’:
cgcreate.c:81:17: warning: ‘strncpy’ specified bound depends on the length of the source argument [-Wstringop-truncation]
   81 |                 strncpy(slice, cg->name, len);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cgcreate.c:80:23: note: length computed here
   80 |                 len = strlen(cg->name) - strlen(scope);

fix it by using, length of destination in the strncpy().

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>