turnip: Put VK_KHR_external_fence_fd stubs back

tu_ImportFenceFdKHR is used by tu_AcquireImageANDROID, which may or
may not work, but let's at least keep things compiling until somebody
has time to tie up the loose ends on the Android side.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5670>