Cast double -> float fixing compile error in OSX

I am not sure the reason, but I was having the following compile error in OSX 10.10:

```
/Users/javier/Dev/PartsBasedDetector/src/HOGFeatures.cpp:116:29: error: invalid operands to binary expression ('Size_<float>' and 'double')
                resize(im, scaled, imsize * (1.0f/pow(sfactor_,(int)i)));
```
Clang version:
Apple LLVM version 6.0 (clang-600.0.54) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.0.0
Thread model: posix

OpenCV 2.4.9