Enable the wlroots' Vulkan renderer if a suitable driver is being built. Currently only Mesa provides Vulkan drivers, so gating the feature on BR2_PACKAGE_MESA3D_VULKAN_DRIVER seems reasonable at the moment (and is done at least by one other package already: pipewire). Signed-off-by: Adrian Perez de Castro <aperez@igalia.com> Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>