package/qt5/qt5virtualkeyboard: install shared library

package/qt5/qt5virtualkeyboard: install shared library

Between Qt 5.11 and 5.12 the virtual keyboard package gained a new
top-level library, libQt5VirtualKeyboard.so, which is then used by all
of the plugins it installs. This change makes Buildroot aware of the
new library so that we install it to the target filesystem.

Signed-off-by: Mark Thompson <mark.thompson@starleaf.com>
[Thomas: move the definition of QT5VIRTUALKEYBOARD_INSTALL_TARGET_LIBS
into a separate condition]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>