Update rrd.inc

Fix this error
php: rc.bootup: The command '/usr/bin/nice -n20 /usr/local/bin/rrdtool update /var/db/rrd/system-mbuf.rrd N:U:U:U:U:U' returned exit code '1', the output was 'ERROR: expected 4 data source readings (got 5) from N:U:U:U:U:U'
Merge pull request #726 from N0YB/patch-2

Update rrd.inc