cmake: Prevent redefinition of uninstall target
cmake: Prevent redefinition of uninstall target

Allows for this repo to be included as a submodule of another
repo that defines its own uninstall target.  The uninstall target
is best defined at the top-level repo.

KhronosGroup/Vulkan-Headers,KhronosGroup/Vulkan-Headers,KhronosGroup/Vulkan-Headers