fix glb_run type for disktest

disktest/main.c and disktest/timer.c fail to compile on certain
platforms due to mismatch in types -- short vs long -- with a
gcc error "relocation truncated to fit"

Thanks to Kyle McMartin for the hint on how to fix the error!

Signed-off-by: Jeffrey Bastian <jbastian@redhat.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>