build: resolve compiler warning from cgroups blkio code

Remove some left-over unused code from an earlier blkio
implementation, causing this with recent gcc variants -
cgroups.c: In function ‘read_blkio_devices_stats’:
cgroups.c:744:16: warning: variable ‘result’ set but not used [-Wunused-but-set-variable]
     __uint64_t result = 0;

Extend the gitignore file for the proc PMDA while there.