link msvfw32 instead of vfw32

On my system (i686-w64-mingw32 cross compiler using mxe.cc) vfw32 is called "libmsvfw32.a".

On a windows build box with MSVC 12.0 the library is also called "msvfw32.Lib".
I am not sure whether MS did rename the library or what's going on here.

OpenCV seems to use a "if(MINGW64)" check, which didn't work in my case:
https://github.com/Itseez/opencv/blob/da1ac359304df7e3a933e83babc5be5f49bc48a9/cmake/OpenCVFindLibsVideo.cmake