Update sgminer.c

One small change to work well with previous ones.
Number of warnings now is 151, down from 157.

Still a long way to go. Most of the serious warnings are related to variables in miner.h, which is so intertwined with everything else that I fear of breaking something or making performance worse if I touch miner.h too much.
Merge pull request #189 from Bllacky/patch-7

Update sgminer.c