Fix math library on Visual Studio.
Fix math library on Visual Studio.

The code was compiled but optimized, and no math function
was used.

Change-Id: Ib2ab262c072761ec41df8f9260a1e0fe57d01cef

PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp