libvulkan: update code gen templates

Update code-generator.tmpl to match the result from doing `$ apic
template ../api/vulkan.api code-generator.tmpl`.
Update null_driver.tmpl to match the result from doing `$ apic template
../api/vulkan.api null_driver.tmpl`.

Test: build
Change-Id: Idcf6c148fd0d49bb1b6a31e657e82970f52923c0