clarify the type of 'warmup' struct member

Initially the warmup field of the checker struct contained number of seconds
and was typed as unsigned int. Then it was changed to store timer ticks, but
the type was not changed to long as for other variables storing timer ticks.

This commit fixes this minor arch-specific issue.
Merge pull request #87 from andriyanov/fix_warmup_type

clarify the type of 'warmup' struct member