zink: do not require VK_KHR_external_memory

zink: do not require VK_KHR_external_memory

This is only required for the DRI-path. For the swrast code-path, we
don't need this.

We also don't need to explicitly test for it in the DRI-path, because we
test for KHR_external_memory_fd, which depends on KHR_external_memory. So
no implementation will expose the former without the latter.

Fixes: f1432fd3e2d ("zink: generate extension infrastructure using a python script")
Reviewed-by: Hoe Hao Cheng <haochengho12907@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7882>