redis plugin: fix build error on NetBSD

redis plugin: fix build error on NetBSD

Making all in daemon
  CC       redis_la-redis.lo
redis.c: In function 'redis_handle_info':
redis.c:307:5: error: array subscript has type 'char' [-Werror=char-subscripts]
     for(i=0;(*str && (isdigit(*str) || *str == '.'));i++,str++)
     ^
cc1: all warnings being treated as errors
*** Error code 1

Stop.
make[3]: stopped in /home/ruben/src/collectd/src
*** Error code 1

Stop.
make[2]: stopped in /home/ruben/src/collectd/src
*** Error code 1

Stop.
make[1]: stopped in /home/ruben/src/collectd/src
*** Error code 1

Stop.
make: stopped in /home/ruben/src/collectd