Fix clang 11 -Wformat warnings in yp_mkdb:

usr.sbin/yp_mkdb/yp_mkdb.c:91:40: error: format specifies type 'char *' but the argument has type 'void *' [-Werror,-Wformat]

printf("%.*s %.*s\n", (int)key.size, key.data, (int)data.size,
        ~~~~                         ^~~~~~~~

usr.sbin/yp_mkdb/yp_mkdb.c:92:7: error: format specifies type 'char *' but the argument has type 'void *' [-Werror,-Wformat]

data.data);
^~~~~~~~~

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