okular: Fix linking with MSVC 2015.
Review Request #125741 - Created Oct. 21, 2015 and submitted
okular/part.cpp includes interfaces/viewerinterface.h and its OKULARCORE_EXPORT macro gets expanded into __declspec(dllimport). This makes VS linker to drop ViewerInterface::~ViewerInterface() definition and to look for it in KF5OkularCore.dll. But this include is never used anywhere in okularcore, hence missing symbol. Including interfaces/viewerinterface.h in any of okularcore sources fixes the problem.