[krbdev.mit.edu #3913] krb5-1.5: fix warnings in lib/crypto/des/string2key.c

[Public Submitter via RT]

fix warning:
string2key.c: In function 'mit_des_string_to_key_int':
string2key.c:229: warning: pointer targets in passing argument 1 of 
'mit_des_cbc_cksum' differ in signedness