vkquake: Fix build when having no X11 deps

[manu]

Summary

When building with no X11 in all the ports vkquake can't find the vulkan_core.h
header. This is because no dependency add /usr/local/include to the CFLAGS.
Fix this by adding setting VULKAN_SDK to ${LOCALBASE}, the Makefile in the Quake
directory will add the proper CFLAGS.

Test Plan

poudriere testport with :
devel_sdl20_UNSET= VIDEO_X11
devel_vulkan-tools_UNSET= XCB XLIB
graphics_mesa-dri_UNSET= PLATFORM_X11
x11-wm_sway_UNSET= X11
x11-toolkits_wlroots_UNSET= X11