Fix linking with okularpart.dll when using MSVC.
Review Request #125792 - Created Oct. 25, 2015 and submitted
To link with DLL using Visual Studio linker, it should be fed with .lib file corresponding to .dll being linked. For libraries created with SHARED keyword, CMake figures this out and passes correct file. But for okularpart target, the DLL itself is passed, which leads to error during linking.
This seems to be a bug in CMake and this patch works it around.