ninfod: Use unsigned int for digest.

Fixes following warning:
| ninfod_name.c: In function 'nodeinfo_group':
| ninfod_name.c:270:2: warning: pointer targets in passing argument 1 of 'MD5_Final' differ in signedness [-Wpointer-sign]
| /usr/include/openssl/md5.h:113:5: note: expected 'unsigned char *' but argument is of type 'char *'

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>