The issue here I believe is the Vulkan runtime being compiled without xlib support. Not sure which distribution you're using, but I know the one that ships with Ubuntu is like this. If you download the SDK from LunarG and use this version instead, it won't have this problem.
Alternatively, if you want to use XCB instead, what you have done is correct, you're just missing the linkage with libX11-xcb. To specify this, open Source/Core/VideoBackends/Vulkan/CMakeLists.txt, and add "libX11-xcb" without the quotes to the target_link_libraries line, after glslang.
The reason we don't do this is because the backend gets passed a XLib handle, and I'm not sure assuming it is a wrapped xcb handle is the best idea.
Alternatively, if you want to use XCB instead, what you have done is correct, you're just missing the linkage with libX11-xcb. To specify this, open Source/Core/VideoBackends/Vulkan/CMakeLists.txt, and add "libX11-xcb" without the quotes to the target_link_libraries line, after glslang.
The reason we don't do this is because the backend gets passed a XLib handle, and I'm not sure assuming it is a wrapped xcb handle is the best idea.