Add missing declaration of PVD in MaliitKeyboard::AbstractBackgroundBuffer
Add missing declaration of PVD in MaliitKeyboard::AbstractBackgroundBuffer

RevBy: Jon Nordby, Jan Arne Petersen
Got lucky so far that the linker was able to cope with the missing d'tor
declaration, but when trying to reuse libmaliit-keyboard-plugin.so, this issue
showed up again.

RHawkeyed/plugins,RHawkeyed/plugins,develersrl/maliit-demo-qml,develersrl/maliit-demo-qml,maliit/plugins,maliit/plugins,develersrl/maliit-demo-qml,RHawkeyed/plugins,maliit/plugins,KDE/plasma-maliit-plugins,develersrl/maliit-demo-qml,RHawkeyed/plugins,KDE/plasma-maliit-plugins,KDE/plasma-maliit-plugins,maliit/plugins,KDE/plasma-maliit-plugins,RHawkeyed/plugins