qt5virtualkeyboard: fix target install issue with Qt 5.6

qt5virtualkeyboard 2.0 (shipped with Qt 5.6) installs the QML plugin
under the directory Enterprise while the later versions install it
without that directory.

Differentiate installation to target of the QML plugin according to
the Qt major version in use.

Also, make sure the /usr/qml/QtQuick directory exists before the copy.

Fixes:

	>>> qt5virtualkeyboard 2.0 Installing to target
	mkdir -p /home/gportay/src/buildroot/output/target/usr/lib/qt/plugins/platforminputcontexts
	cp -dpfr /home/gportay/src/buildroot/output/host/arm-buildroot-linux-gnueabi/sysroot/usr/lib/qt/plugins/platforminputcontexts/libqtvirtualkeyboardplugin.so /home/gportay/src/buildroot/output/target/usr/lib/qt/plugins/platforminputcontexts
	cp -dpfr /home/gportay/src/buildroot/output/host/arm-buildroot-linux-gnueabi/sysroot/usr/qml/QtQuick/VirtualKeyboard /home/gportay/src/buildroot/output/target/usr/qml/QtQuick
	cp: cannot stat '/home/gportay/src/buildroot/output/host/arm-buildroot-linux-gnueabi/sysroot/usr/qml/QtQuick/VirtualKeyboard': No such file or directory
	package/pkg-generic.mk:310: recipe for target '/home/gportay/src/buildroot/output/build/qt5virtualkeyboard-2.0/.stamp_target_installed' failed
	make: *** [/home/gportay/src/buildroot/output/build/qt5virtualkeyboard-2.0/.stamp_target_installed] Error 1

Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>