rrd_info: make filename argument const

Avoids following error:

rrdinfo.cc:10:42: warning: deprecated conversion from string constant to ‘char*’ [-Wwrite-strings]
   rp = rrd_info_r("/home/pab/ted5000.rrd");

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Merge pull request #518 from pabigot/fix-rrd_info-param

rrd_info: make filename argument const