vkquake: Fix build when having no X11 deps

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.

Approved by: Greg V (maintainer)
Differential Revision: https://reviews.freebsd.org/D25233