src/meson.build: Look for Vulkan as well

Hi,

This attempts to fix builds against GTK that has Vulkan support enabled, as the GTK headers pull in the Vulkan headers in this case and the GTK dependency only pulls in the Vulkan dependency only when Vulkan was found via pkg-config when GTK was being built (which is typically not the case outside of *nix platforms, from the stock LunarG Vulkan SDK).

In this case, as we have no way to query the GTK dependencies to see whether Vulkan is enabled, make an optional dependency on Vulkan and use it if it was not found via pkg-config, so that we can find Vulkan's headers during the build, so that the builds won't break.

With blessings, thank you!