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

games/vkquake/Makefile

@@ -25,7 +25,7 @@ GH_PROJECT=	vkQuake
 
 INSTALLS_ICONS=	yes
 
-MAKE_ENV=	prefix=${PREFIX} bindir=${PREFIX}/bin
+MAKE_ENV=	prefix=${PREFIX} bindir=${PREFIX}/bin VULKAN_SDK=${LOCALBASE}
 ALL_TARGET=	# empty
 
 .include	"${.CURDIR}/../quake-data/Makefile.include"