Misc. bug: Vulkan is not optional at runtime

[daym]

Name and Version

With llama.cpp version from git tag b4549, if I compile Vulkan support in and then run in an environment where Vulkan is not supported (for example this would happen if a Linux distribution provides llama.cpp with Vulkan enabled but the user doesn't have a GPU with Vulkan), it will fail with the following exception:

terminate called after throwing an instance of 'vk::IncompatibleDriverError'
  what():  vk::createInstance: ErrorIncompatibleDriver

It would be better to just disable Vulkan in this case but run on CPU.

Operating systems

Linux

Which llama.cpp modules do you know to be affected?

libllama (core library)

Command line

Any llama-cli command (yes, even `llama-cli -dev none`).

Problem description & steps to reproduce

1. Compile with GGML_VULKAN=ON
2. Run without GPU
3. It crashes with exception

First Bad Commit

No response

Relevant log output