While procps_diskstat_ref and procps_diskstat_unref were declared
they actually weren't implemented in the library. Linking some
binary that used these functions gave the following errors:
CCLD vmstat
vmstat.o: In function `disksum_format':
vmstat.c:719: undefined reference to `procps_diskstat_unref'
vmstat.o: In function `diskformat':
vmstat.c:591: undefined reference to `procps_diskstat_unref'