kadm5: add -Wno-alloc-size-larger-than

This trips the gcc build in kadm5/srv/svr_principal.c:

/home/ivy/src/bsd/gcc/crypto/krb5/src/lib/kadm5/srv/svr_principal.c: In function 'apply_keysalt_policy':
/home/ivy/src/bsd/gcc/crypto/krb5/src/lib/kadm5/srv/svr_principal.c:208:14: error: argument 1 range [2147483648, 4294967295] exceeds maximum object size 2147483647 [-Werror=alloc-size-larger-than=]

208 |     subset = calloc(n_ks_tuple, sizeof(*subset));
    |              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Differential Revision: https://reviews.freebsd.org/D51577