Fix argument with mismatched bound in isc_iterated_hash()

Fix argument with mismatched bound in isc_iterated_hash()

GCC 11 produced the following warning:

    iterated_hash.c:21:33: warning: argument 1 of type ‘unsigned char[20]’ with mismatched bound [-Warray-parameter=]
       21 | isc_iterated_hash(unsigned char out[ISC_SHA1_DIGESTLENGTH],
          |                   ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
    In file included from iterated_hash.c:18:
    ./include/isc/iterated_hash.h:33:37: note: previously declared as ‘unsigned char[155]’
       33 | int isc_iterated_hash(unsigned char out[NSEC3_MAX_HASH_LENGTH],
          |                       ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~