runtime: Add a new IcdKind for Vulkan meta-layers
Vulkan meta-layers doesn't have the library_path
field in their
manifests.
For this reason we need to separately handle them because otherwise our
assertions about library_path
not being NULL will fail, like the one
in collect_vulkan_layers()
.
This fixes the issue reported on https://github.com/ValveSoftware/steam-runtime/issues/306
/cc @smcv