Can't build sysutils/xentools420

[Martin Husemann <martin%duskware.de@localhost>]

The build of sysutils/xentools420 on NetBSD-current amd64 from pkgsrc-current
stops for me with:

In file included from ../qemu-xen/ui/sdl2.c:31:
/work/pkgobj/sysutils/xentools420/work/xen-ea5025554e982bd377f85b899b6d151e5ef33d1e/tools/qemu-xen/include/ui/sdl2.h:7:10: fatal error: SDL.h: No such file or directory
    7 | #include <SDL.h>
      |          ^~~~~~~
compilation terminated.


Martin