Visual Studio builds: Include Vulkan sources in GSK
...but disable them for now. Configs will be added for the projects to support Vulkan-enabled builds which will then enable the builds of these sources. Extra commands and items will be needed for the GSK resources along with ensuring GSK_RENDERER_GSK being defined for the build of GDK, GDK-Win32 and GSK so that the builds of Vulkan-enabled builds can be done properly. Filter out the Vulkan sources from the 'dist hook' rules in gsk/Makefile.am as we don't want to in turn include them twice in the projects when the 'make dist' is performed on a system with Vulkan builds enabled.