Fix okularplugin linking on VS 2015.
Review Request #125742 - Created Oct. 21, 2015 and submitted
okularplugin target from mobile/components suffers from the same problem as okularpart in https://git.reviewboard.kde.org/r/125741/
But the cause is a bit different now. okularplugin uses settings.cpp generated by KConfig and its header declares Settings class with OKULARPART_EXPORT. For okularplugin target this macro gets expanded to __declspec(dllimport) and it makes linker to look for Settings class symbols in okularcore.
Fix this by tricking VS that we are building okularpart target.