QtGui: Vulkan: Hide Vulkan SDK headers from public Qt headers

Currently QtGui's public Vulkan headers such as qvulkaninstance.h contains the header files from the original Vulkan SDK, if the user includes such Qt headers but not use them, he will always get a compilation error if his project does not link against the Vulkan SDK package. And in this case using #QT_CONFIG(vulkan) will not help because the official Qt packages always build with Vulkan support but no vulkan headers are shipped at the same time. It's really inconvenient for users who are developing graphics applications and want to support multiple backends but may not install the Vulkan SDK. So I suggest to hide Vulkan SDK headers from public QtGui headers to avoid such problems.