api.c: Fix strncpy() truncation warning

api.c: Fix strncpy() truncation warning

Fix the following strncpy() string truncation warning:

In function ‘strncpy’,
    inlined from ‘cgroup_parse_rules_file’ at api.c:676:3:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:106:10: warning: ‘__builtin___strncpy_chk’ output may be truncated copying between 0 and 255 bytes from a string of length 4352 [-Wstringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>