package/qt6/qt6base: use correct QT6BASE_CONF_OPTS variable for Vulkan

The Vulkan option was appended to QT6BASE_CONFIGURE_OPTS instead of
QT6BASE_CONF_OPTS, which is the variable actually used during CMake
configuration. This prevented the feature from being enabled/disabled
as expected.

Fixes: 1c27f3a1 ("package/qt6base: add vulkan option")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>