Fix okularplugin linking on VS 2015.

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 OKULARCORE_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 okularcore target.

REVIEW: 125742