libktx: install format headers?

I'm learning the API for `libktx`, and according to the [usage overview](https://github.khronos.org/KTX-Software/libktx/index.html#subsection) for writing a ktx1 or ktx2 file, I would need to include the following headers:

```
#include
#include
```

However, `vk_format_enum.h` does not appear to be installed by the CMakeLists.txt (I'm using conan to install ktx, but I believe this is also the case if building and installing from source). There are a couple of other headers, like `vk_format.h` and `gl_format.h` that may also be useful to have installed.

By the way, while I could of course include the system vulkan and opengl headers, the piece of software I'm writing is intended to not have a dependency on these so it would be nice to have these stub headers provided by the library.

Start by reading the CMakeLists.txt install rules and compare the headers used in the libktx usage overview with those included in the installed package. Verify a source installation and the Conan package expose the requested format headers without requiring system Vulkan or OpenGL headers; done means consumers can include the supported headers successfully.

Written by the indexing model from the issue text.