api.c: Fix strncpy() truncation warning

api.c: Fix strncpy() truncation warning

Fix the following strncpy() string truncation warning:

In function ‘strncat’,
    inlined from ‘cgroup_get_cgroup’ at api.c:3153:3:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:136:10: warning: ‘__builtin___strncat_chk’ output may be truncated copying between 0 and 4095 bytes from a string of length 4095 [-Wstringop-truncation]
  136 |   return __builtin___strncat_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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