VS2010 build: fix conditional define in opt_set_floatval().

As per https://github.com/veox/sgminer/commit/5ce16a44db3c56730f13f564f88e1d5a3b775619#commitcomment-5363058
Merge branch 'build-msvs2010-upd' of github-veox:veox/sgminer into build-msvs2010-upd