utils/libav: test for <vulkan.h> dynamically

Now that we bundle <vulkan.h>, PL_HAVE_VULKAN is no longer a "reliable" method of detecting whether vulkan is enabled. To be fair, it never was, but this change revealed this bug.

The real issue here is that <vulkan.h> cannot be unconditionally assumed to be available, so we need to test for it somehow. Re-introduce the dynamic check (works on GCC/clang) and leave a scary note hoping to get MSVC users to include <vulkan.h> manually.